Why not include all of the routines, set aside enough space in the
"real" routines for the largest of all the possible routines, which is
just full of nops, and self modify that code space with the routine
you want to use?  Zero overhead execution time wise.  If space is a
concern, put each possible routine in a page aligned full page of
space, once the routine you want has been copied into it's final
destination you can give back all that space to the pager.