I have a similar issue. Rolling out UTF8 support everywhere is better done sooner rather than later I think. From what I can see, Exim supports SMTPUTF8, so perhaps we'd all be better off if it was enabled in the default Exim config?
Edit: It looks like support for SMTPUTF8 in Dovecot/LMTP is limited at best, so until the entire stack fully supports SMTPUTF8, no solution is ideal. Personally I've opted to not advertise SMTPUTF8 support as early in my own stack as possible, which really just moves the problem. However, I think rejecting a mail outright, is better than bouncing it later.