Thanks for looking into it. It can't be easy on a 'Labs' feature as it must be liable to change.
A couple of thoughts about how I'd hope it would work (better to ask before you implement it rather than after)...
1 - If it comes down to the choice of using the Google icons or a user's own, then I'd very much prefer to use my own (I rarely view my calendar via Google, I usually see it via Ranlender).
2 - Ideally, I'd like them to appear in the same way as category icons, i.e. at the start of the line in the event window and (optionally), in the calendar window.
3 - Oh, and in colour too please rather than the grey scale of the category icons.
In case it's of any use for testing, I created a set of icons some years ago for use with the mobile calendar software I use (called AquaCalendar). I would hope to be able to use them (or some version of them) with Rainlender and add to them as and when I require a new icon.
If they're of any use to you, feel free to make use of them. The final ones are the 12x12 versions available here
. The cyan backgrounds are meant to be transparent. If you need them in some other format, just let me know.