The latest photos on Chris Broderick's myspace shows him playing his Ibanez 7-string with Megadeth on their finland show last Feb 4! Now I just wonder:
1. Would there be Megadeth songs were he (secretly) employs that low B? That would surely bring out some new twists to the 'deth sound. Would Dave allow that?

No. Dave only appreciates the high b. in fact, the pictures we've seen of chris on stage with megadeth have been with with an added high b as the 7th.
2. Would he introduce 7 string riffage to Megadeth in the next album?

it's hard enough for dave to allow his hired guns to bring in 6 string riffage. either way, dave rarely writes riffs that require more than 3 or 4 strings. usually D,A, & E.
3. Would Chris eventually get his signature Dean 7 string? (assuming he stays longer than Glen Drover!). Only if they agree to putting Dime's graphics on the body.