Schumann lab from Uni. Beyreuth, DE have done some interesting work on using spores to direct antigens to the gut - to act as vaccines. Sporulation guys might also be interested in this paper, describing the B. sub coat protein (and how it's hilariously complex but not all required). Oh, and subtilis spores willgerminate in the gut (probably), justifying using the killswitch!
Killswitch guys, I think perhaps looking into recombinases as opposed to restriction enzymes would be useful as they won't act on host DNA. Xer and Dif sites will recombine with themselves in presence of the required enzyme, excising any genes between them - you could flank genes with them, then express the enzyme to chop your construct up. Sites are required to be within ≈5kb of each other, I think, so random ones on host DNA shouldn't be affected. It might take a while to work so look into the time; could be useful as a fallback, anyway.