Given the size and weight of the "howdah", those straps would be BIG, probably a double-thickness of elephant leather (4-5 inches thick) and 2-3 feet wide. Given how much effort it takes to cut a normal leather belt with a knife, just hacking once or twice with a sword won't be enough; they'll have to saw through each strap. Call it hardness 2, 80-100 hit points per strap. The illustration shows at least five major straps, and they're cross-connected by smaller ones, so to destabilize the howdah the party will have to sever at least three straps, probably four. You mentioned metal reinforcement, but overlapping plates of thick wood are more likely: the gnolls have a druid who can cast Wood Shape, making wooden plates much faster and easier to produce than metal ones - and if there's anything a gnoll hates, it's having to do extra work. Figure each plate is like a large wood shield.
Just getting up to the beast's belly to get at the straps will be fun in itself (roughly ten DC20 Climb checks). While they're hacking away, of course, the wyverns will be trying to pick them off, like tickbirds on a rhino. The gnolls will probably lower two archers in the transport cage to snipe at the PCs once the first strap has been cut and the crew realize what is going on. The elite, flaming-maned gnolls may even rappel down the side on ropes to fight the party. The H'Jyord, the wizard Sylkess, and the gargoyle Seigonas can all fly, and Gnu-Dalcom could Wild Shape into a flying form instead of a crocodile, so all could harass the party (probably all at the same time, since Sylkess with her Int 19 probably realizes this is better than allowing themselves to be beaten piecemeal).
