I solved it - after some work. I created a custom generator, posted below in case anyone else needs something similar.

The pattern matching in particular was troublesome to figure out. "egrep style regexp", as the docs put it, didn't say much; particularly regarding the way MATCH divides its matched strings into different elements of the return value. Perhaps there is a nicer way to deal with prefixes in bjam?

# omniidl.jam

import generators ;
import type ;
import "class" : new ;

type.register IDL : idl ;

class idl-generator : generator
{
    # Determine the name of the produced target from the
    # names of the sources.
    rule determine-output-name ( sources + )
    {
        # The simple case if when a name
        # of source has single dot. Then, we take the part before
        # dot. Several dots can be caused by:
        # - Using source file like a.host.cpp
        # - A type which suffix has a dot. Say, we can
        #   type 'host_cpp' with extension 'host.cpp'.
        # In the first case, we want to take the part till the last
        # dot. In the second case -- no sure, but for now take
        # the part till the last dot too.
        name = [ utility.basename [ $(sources[1]).name ] ] ;

        for local s in $(sources[2])
        {
            local n2 = [ utility.basename [ $(s).name ] ] ;
            if $(n2) != $(name)
            {
                error "$(self.id): source targets have different names: cannot determine target name" ;
            }
        }

        # Names of sources includes an I. Strip it!
        name = $(name:D=) ;
        #match the first letter I. Put it in m[1].
        #Then match everything else. Put it in m[2].
        local m = [ MATCH (^[I]?)(.*) : $(name) ] ;
        return T$(m[2]) ;
    }
}

generators.register
[ new idl-generator omniidl.omniidl : IDL : CPP(%Caller) CPP(%Dispatcher) ] ;

rule omniidl ( targets + : source : properties * : d * : e * )
{
}

#Hrrrmmm... make this nicer. And a LOT more general.
actions omniidl
{
<idl invocation command> -C ./ $(>)
cp *.h bin/gcc/xxx/link-static
cp *.cpp bin/gcc/xxx/link-static
}

# end of omniidl.jam