I've absolutely run into this. The more engaging volunteer opportunities and potential part time employeers seem want you to commit to a schedule that excludes the flexibility that I FIRED for. When you consider the craziness of my kids' school schedules (random teacher work days, snow days, events, sick days, voting days, etc), any attempt at a firmly scheduled work/volunteer day of the week may result in 1-3 cancellations or reschedulings a month. Not to mention it still sucks spending that absolutly perfect day for hiking, surfing, kiting, boating, etc tied down with some prior commitment.
I guess it's just another example of most of the world wanting you to conform to their box and not understanding what it means to be free.
Over the two years I have been retired, I have managed to "train" my two primary volunteer/pt work "managers" to accept what I am willing to give them, but there was/is certainly a bit of friction.
I haven't quite adopted this method yet, but one way to approach to the issue would be to commit blocks of 6, 12, 24, etc months at a time to a cause, then when the period is up, move on to the next item of interest.
When you are talking about several decades of retirement, you could accomplish a rediculous number of things with this method, maybe just not simultaneously. I do have to admit that when I first retired, I might have been in a rush to catch up on all the things I had on the back burner while working, but have since relaxed a bit, realizing I (should) have better than half a lifetime to do all the things I want.
Anyway you slice it, it's still better than getting up every morning and dragging yourself into the same old office!