Sometimes stats just don't work like you'd expect them too. According to retrosheet.org's situational splits, during the seasons from 1988 through 2005 (excluding 1999, because they don't have it yet), batters were hit by pitches once every 139.9 plate appearances with no runners on base, once per 115.4 plate appearances with runners on base, once per 106.4 plate appearance with runners in scoring position and once per 100.6 plate appearance with the bases loaded. The rate of batters getting hit in bases loaded situation is 39% higher than their rate with the bases empty, but given the relative rarity of bases loaded situations, RBI plunks only account for 2.3% of total HBPs.
Craig Biggio's situational numbers for that span follow a similar pattern. Over those same years (1988-2005 excluding 1999) he was hit once per 44.1 plate appearances with the bases empty, once per 35.5 PAs with the runners on base. He was hit once every 33.6 plate appearances with runners in scoring position, and once per 25.1 plate appearance with the bases loaded. He had 7 bases loaded plunks in 176 plate appearance in those years. (He hasn't had an rbi plunk this year, and didn't have one in 1999 either, so 7 is his complete rbi plunk total, but I do not have his complete career bases loaded plate appearances total). Biggio's increased rate of getting hit when the bases are loaded is even more pronounced than the leaguewide totals. He has gotten hit 75.5% more per plate appearance with the bases loaded than with the bases empty. But, he's had about 38 times more plate appearances with the bases empty so the vast majority of his plunks have come in that situation. (Interestingly, all 7 of his plunks this year have come with runners on base)
During those same years since 1988 that are fully accounted for on Retrosheet, there have been 18 game winning walk off HBPs. There is a complete list of them back to 1958 on baseball-reference.com's blog here. Craig Biggio is not on the list yet, so there are some feats of getting hit that have eluded his long career.