I'm not sure I believe you when you say "there are no relevant milestones". From a business stand point it doesn't matter what technical milestones are hit, but almost every business in the world tries to determine if employees are doing as well as expected or better and there is often no clear link to the exact monetary value of any given dev work. There are obviously issues here, but I find it hard to believe any compensation structure exists that doesn't, if nothing else but because teamwork is hard to quantify.
If this project isn't expected to take more than a year to get off the ground, it doesn't sound significantly different from just having a very uncertain bonus.
In any case, if you have no preference in how they should be compensated, consider letting them decide how they would like to be compensated; in a bonus driven culture people may be quite happy to take a percentage they think is reasonable, or they may want to keep their existing structure.
If these developers are able to command such a salary elsewhere, I doubt you will have much luck getting them to take a salary structure they dislike simply because the stakes are so high.