I wouldn't doubt they discussed this before coming to an agreement.
It's probably impossible to estimate how transparent Frost and his staff are with one another, especially when it comes to their career choices. I think that a lot of fans assume that the hiring/firing process for Frost's staff looks something like what you'd expect at your place of work, but they don't operate that way. When they were at UCF, and he started getting job offers, when they had staff meetings he would wait until the end of the meeting then lay out whatever offers or inquiries he was receiving, and they would then discuss as an entire staff the pros and cons of each potential hire. It was ALWAYS assumed by Frost that he would ONLY take a job if he could bring his whole staff, but it's also understood by all parties that each coach needs to make whatever decisions are best for him. They even discussed the dynamics of how hiring certain coaches would affect their families because the coaches' wives are each other's support network, so they want to make sure that the wives click with one another also. They don't have to be best friends, but if a top-notch coach has a reputation as a ladies' man, or if his wife likes to bring the drama, Frost isn't hiring that coach. The long-term effects aren't worth it for the short-term tradeoff.
Before Frost was ever hired at Oregon he had had discussions with Mike Bellotti (who was actually the head coach of Oregon at the time of the hiring process) and Chip Kelly about what his career aspirations were, what he'd like the timeline to look like, etc. He was actually a defensive coach at UNI at the time, but the Oregon coaches told him that he'd be able to climb higher more quickly at Oregon on the offensive side, which was why they encouraged him to make the change. He had no special association with being a WRs coach, but they could recognize that he had a brilliant mind, that he was hard-working, and that he could easily connect and build relationships with a wide variety of people,... and he was relatively low maintenance. That's what Bellotti and Kelly wanted, and they said that they'd figure out the best fit for him eventually, but they mostly just wanted to get him on staff. Before he was even hired as a WRs coach, Bellotti was already looking ahead to the move that he was about to make from Head Coach to AD, which meant promoting Kelly from OC to Head Coach, which meant promoting Helfrich to OC, and they already had in mind that Frost would follow Helfrich. Frost thinks the same way. That affects how the whole staff views themselves.
I haven't seen it reported by any media folks, but there were rumors that Greg Austin wasn't very happy in Lincoln this past season. If that's true, he wasn't going to go sulking around and plotting what to do next in private. He would have gone to Frost and plainly told him what was upsetting him. If that was true--and I see lots of reasons to suspect that it was--Frost's promotion of him to running game coordinator was Frost's way of both solving a team problem--Austin could do more to improve the overall running game--and the personnel/staff problem of how to help Austin want to stay in Lincoln. Ditto for Held and his title of "Recruiting Coordinator," which I suspect will help Held as he recruits, and it will probably give him a bit more say (possibly even some veto power) in who they offer and go after. Austin cares passionately about the running game not working properly, and Held cares passionately about recruiting, so Frost made moves to help both out so that they could do more with where the passion was, and where it would be for the good of the team.
How does all of this connect to Lubick and how long to expect him to be at Nebraska? I'm saying that I'd be utterly shocked if there are any secrets about what both sides (Frost/Nebraska and Lubick) want and expect to get out of the arrangement. Assuming that Lubick's family continues to live in Fort Collins, that's a blinking neon light to me that both he and Frost have no expectations for him to be working at Nebraska indefinitely, and both sides are more than fine with that. If Lubick's family is still in Fort Collins, not only will Lubick be making frequent trips back there, but he'll be encouraged to do so by Frost, especially as it appears that Lubick has a bit of an OCD tendency that leads to being a workaholic if there aren't good boundaries. The irony is that a lot of folks here seem to think that a coach like Lubick can't do his job if he's not unhealthily laser-focused SOLELY on his job, but the opposite is true: if a coach's family life isn't healthy, it's impossible to be able to focus on your job enough to do it well.
Lubick is going to be a great coach at Nebraska and a great addition to the program. He also won't be in Lincoln 5 years from now, and that's probably ideal for everyone.