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.