you could consider a pseudo-partigyle.
Mash in with your normal bock recipe with mostly munich and some caramunich, run off and then cap the mash with a few lb of pilsner, add water and do a 20 minute mash or so. this should get you in the gravity range but without the intense body of the first runnings.
so first runnings = traditional bock, second runnings + 3-4 lb of pilsner = eisbock.