Well, the bank would definitely check into how encumbered the collateral was (what sort of competing claims/legal impediments to the bank claiming said collateral), but if there would be no apparent legal difficulty with them claiming the collateral in the event of a default, they would undoubtedly take that offer... and be looking for opportunities to offer additional such services to the noble in question, particularly if they could all get such generous guarantees. (A 50% premium in the event of a default? Even if the bank had to sink a couple of megacredits into legal fees to secure their guarantee, they'd still come out well ahead. Good business, from their standpoint. And if the noble regularly makes such offers, he's an idiot, financially.)
Kinda reminds me of the old, old exchange - how do you make a small fortune in <risky endeavor>? (Answer: start with a large fortune.) This noble is apparently a trust fund baby turned "gentleman adventurer"... and a rational noble clan looking to maintain and/or build its status for the future would probably be looking at making him a remittance man. Much safer than allowing him to stay anywhere around policy-making!