I'm stuck with using Qmail which has no LMTP support, and thus I'm using dovecot-lda which has certain drawbacks.

Has anyone found a way to direct dovecot-lda to deliver the mail to the LMTP server or any other way for Qmail to deliver the mail to the LMTP server directly ?

Just a thought off the top of my head: you could perhaps pipe the mail to exim instead of dovecot-lda and that can easily be configured to deliver to LMTP