nmh 1.6 added a "-build" option to comp(1). We could use that for composing new messages, rather than the mechanism that went into MH-E 8.6.
My only hesitation is that we still need to support nmh 1.5, and that mechanism should also work for nmh 1.6 and later versions. So changing MH-E to use -build with comp(1) could just be adding complexity for little gain.
Mailutils-3 will have support for "comp -build" (it's already available via the beta releases).
It looks like mailutils 2.x also supports "comp -build", though I haven't tested it.
I plan to address this at the same time as #485.