How do I post an associateLead call and then submit a form?

I want to post form data to Marketo before submitting it to an existing system. It seems that there is no way to tie into the asychronous creation of the tracking pixel, so synchronous form submissions cause the associateLead call to be abandoned.

Is there any callback facility built into the mktoMunchkinFunction function?

I tried overriding the _MchHlpr.doImageGet function after Munchkin loads, but that seems to get overwritten at some point before I actually post to the endpoint. I can keep trying but would love to know if someone has already solved this.