(define Ω (let ([w (lambda (f) (f f))]) (w w)))

Hm, I think I see how to do that. rot13 for spoilers:

Gb pbafgehpg gur fvqr bs lbhe fdhner, lbh arrq gur trbzrgevp zrna bs gur fvqrf bs lbhe erpgnatyr.
Va nal pvepyr, vs pubeqf no naq pq vagrefrpg ng r, gura nr gvzrf ro rdhnyf pr gvzrf rq. V'z fher gurer'f n anzr sbe guvf cebcregl ohg V qba'g erzrzore vg.
Gurersber, sbe lbhe erpgnatyr jvgu fvqrf k naq l, pbafgehpg gur pvepyr jvgu qvnzrgre k cyhf l, naq qenj n crecraqvphyne pubeq guebhtu vg yvxr n puevfgvna pebff. Gur arj pubeq vf qvivqrq vagb gjb rdhny frtzragf bs yratgu fdeg bs k gvzrf l. Qbar!