I don't really see why such a side deal would be possible - if the VC goal is to get the founder-manager to try for that narrow hope of few percent of major suc

I don't really see why such a side deal would be possible - if the VC goal is to get the founder-manager to try for that narrow hope of few percent of major success and discourage them from settling for a lower-value stable business, allowing the founders to cash out would be counterproductive, it's in the VCs interests to ensure that founders personal financial motivation is aligned to theirs, that the founders are also motivated to go big or go bust. In essence, if the startup is slowing down and perhaps not going anywhere, then the standard existing deal with founders where the founders can cash out only if they enable to VCs cash out at a profit (for example, if they manage to pull out all the stops to make a failing startup look attractive to some bigger fool) is exactly what VCs want, and in such a situation the founders have no leverage to extract a compromise that's worse for VCs.