I don't think so, but you can ask Thunderbird if they are willing to earmark your donation for JMAP specifically:
> Who can I email directly with questions about giving?
> If you have a question about giving to Thunderbird, please contact us via this form. We will do our best to follow-up with you as soon as we can. If you need technical support, please head over to Thunderbird Support for assistance.
https://www.thunderbird.net/en-US/donate/
You can also try contacting the developer who offered to work on JMAP integration for pay:
> I can't commit to a project of this size however since I work freelancing and my availability is fluid. I am open to being sponsored/contracted in order to do this as part of my job.
https://bugzilla.mozilla.org/show_bug.cgi?id=1322991#c23