Proceedings-level paper but no proceedings invitation I'm in mathematics.

There's a specific example in a framework my collaborator and I outlined that many people ask me about. It did not fit into previous papers due to space considerations for journals. I feel that writing a note that explains this example in our point of view would be useful for the community.  This example would have a few minor results along the way but mostly it would be a detailed example of our previous work. For that reason, I think it makes most sense in a proceedings; however, I have no proceedings invitations at this point in time.

Should I just sit on the paper or is there another venue or method to deal with this type of issue? Should I put it on the arxiv until a proceedings volume comes along?
First off, "a note [explaining] our point of view" sounds like a perfect use of arXiv, or of a technical report service if your institution offers one.  

There's no delay and no formal procedure necessary, and if it turns out to indeed be useful to the community, then it's readily citable.  Furthermore, there's certainly no shame in putting "side" results out in these more informal format, particularly in a community such as mathematics that is generally used to sharing pre-publication materials.

In parallel, if you think it rises to the level where peer review is appropriate, I see no reason to avoid submitting it to an appropriate community journal---no need to wait for an invitation to a particular venue.  

In short: if it's worth reading, it's worth making available, and the scientific community can surely decide if it's finding it useful or not.