Proposed implementation for the requested feature are attached to ticket #33 (mods to build/type.jam and build/virtual-target.jam).   See https://zigzag.cs.msu.su/boost.build/ticket/33

Added a rule type.set-generated-target-prefix that follows the pattern of type.set-generated-target-suffix.

Criticism is welcome from more experienced bb2 developers.  Not sure what documentation would have to be touched in order to adopt this.

Cheers,
Mark