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.
Biobrick images you can use if you need (advisors like named/labelled circuit diagrams!):
Killing guys, things to keep in mind for the restriction enzymes...
Cuts at short sequences; makes it easier to insert (see next point) and will cut in the genome and plasmid more often by chance
Restriction sites be inserted by codon changes; GeneArt optimise constructs to remove restriction sites all the time, so putting them in should be OK
Is not native; if it's expressed somehow by the cell, it'll suicide on its own
Regulation of expression:
Needs to be near 100% off when off; even a low level of expression will destroy your cells long before any product is produced
You may want to look for a less-efficient enzyme so small leakage wouldn't be that bad; or an enzyme that is degraded relatively quickly
Bistable switch with no leakage might be good; or flippase regulation! My pet idea for the bioremediation project last year =P
You can also insert sites after the transcription terminators in all your genes to ensure full destruction of the construct, and maybe add your sites into normal bricks (terminators etc.) and put them up as variants.