I don't know what Wurtz-class metalloorganyles are, but I assume that you
mean C-C bond formation by activated carbon nucleophiles (Grignard etc).
This proceeds by back side attack with consequent steric inversion. This
is of course impossible with a polycyclic diamondoid target.
What other reactions might work for subunit linkage? Diels-alder won't work
because it requires highly unsaturated reactants, which are not permitted by
the harsh conditions for rearrangement into diamondoid building-blocks.
Radicals might work, though any easily removed radical group would probably
also be destroyed in the building-block phase.
--CarlF