The achievement Herald of the Titans states:
Defeat Algalon the Observer in 10-player mode without anyone in the raid wearing any equipment with an item level higher than is available in 10-player Ulduar.
10-player Algalon loot is all ilvl 239.
Most 10-player HM gear is ilvl 239, and some weapons are 245.
Reading the Wowhead comments, it appears that we can wear Ulduar 25 (i.e. ilvl 232 armor, 10-m HM weapons), and possibly even ilvl 245 weapons for ToGC 10 or Ony.
If anyone has anything to add to or clarify some of the comments on Wowhead regarding this, that’d be great. (especially the first comment which suggests you cannot wear Algalon 10 man items).