> interest from senior engineers who want to work part time
This is definitely of interest. In my experience, companies generally derive significant "senior engineer" value for a handful of things but then their time winds up being consumed by a lot of other "this could have been an email/automation tool/non-senior" tasks. If you are in a position to allow a senior engineer to focus on their significant value adds to the company and the work benefits more from parallelism than what could be had from a single senior engineer (i.e. different lines of work with major context overhead) then you could probably derive more value per senior engineer via part-time work.
A side-benefit that is often not considered is that senior engineers in such a situation may choose to work multiple jobs. Assuming there are no conflicts of interest, chances are this is a net benefit for all. The engineer gets to diversity their start-up equity exposure. Their learnings from any one company can cross-pollinate to others. They can enjoy working on a wide range of problems rather than burning out against one. In this case you may get the value of far more experience than you'd get if they were siloed in one company, not dissimilar from the value that derives from other silo breaking endeavors elsewhere in the field.
> join a team and essentially be a guru or advisor to the team
Honestly, it sounds like you already know what you need: an engineer whose value is technical experience. You may already have enough junior engineers that can get "work" done but what you need is someone who understands the domain, the problem, the desired solution, and can efficiently direct that "work" towards a product while upleveling juniors along the way. Chances are you can hire explicitly for that in part-time and you'd get non-trivial interest.