Currently growthbook doesn't seem to work with custom Mixpanel events - is it possible to support those/some way I haven't seen before?

It should work with custom events. Are you trying to add a metric or something else?

ah hmm, yes a metric

Oh, ok. That's what Mixpanel calls computed events. Yeah, I don't think they expose those when querying with JQL. https://community.mixpanel.com/building-analysis-reports-14/is-it-possible-to-use-custom-events-in-jql-4485

ah unfortunate, it would therefore be good if a metric could be added as "user has done event name A or B or C" etc

Just added an issue for this. This won't 100% support your use case on its own since it looks like you have some conditions that only apply to specific event names. I think we can fix that by allowing custom JavaScript conditions. https://github.com/growthbook/growthbook/issues/432

That would be great - JS conditions. Might need to make sure there's a couple of examples so we can copy and adjust