Which characters do you think would’ve stood up for Dunk at Ashford? Regardless of time period and whether they would be allowed to participate in canon (non knights count). And who would’ve done it out of altruism/taking knightly vows seriously?

Of all time is a bit much, so let’s focus just on ASOIAF characters: 

I think Brienne would without thinking about it. Ser Barristan, post-defrocking, ditto. If it wasn’t a joust and more of a duel/trial by combat, Syrio Forel certainly. Jon probably would, Robb as heir probably shouldn’t. Loras and/or Renly would probably do it, but more for the PR than for the vows. Sandor probably wouldn’t because he thinks knighthood is bunk, but if Gregor was on the other side he probably would. Ned would probably object to the whole concept of a trial by seven – the two men should be the only ones fighting it out. 

Oberyn might do it for the lulz as might Jaime if the Kingsguard thing wasn’t an issue. 

I think Sansa would want someone to stand up for him, but obviously couldn’t do so herself, maybe give him her favor or pray for him. Arya in AGOT would want to, Arya from ACOK on would probably be too survival-focused unless it was closer to the situation with Sam in Braavos. Dany would probably order Belwas or Barristan or Jorah or Daario to be one of his seven.