There are non-domain related things you can do to buff hires and summons as well, of course. Not all will be possible (I have a charmer built specifically for the LHoX raid, where charming an enemy is the key mechanic... some of its buffs are class specific to "not-cleric", but some are universal/available to anyone which I will list below).
Every epic level grants +5 melee and ranged power, +10 universal spell power, and +5 prr/mrr. So that's a given for everyone. TBH I'm not sure if legendary levels count for this or not as this was added before legendary levels existed and I haven't done comparative testing yet.
Spells:
Haste for attack speed (scroll casting as not on the cleric list)
Bull's Strength, Mass (this is on the cleric list, yay)
Tenacious Pack for +30% fortification and +60 Heal Amp (LGS clickie as it's a druid spell, so that means farming a raid for this defensive buff)
The other buff spells specific for hires/summons are druid only and afaik don't have an alternate way to cast.
Universal Enhancement Tree: Harper Agent
* Harper Leadership (+4 to all hire/summon ability scores)
* Heroic Companion (also available via halfling): 1 minute duration, has charges, single ally target: +1[w] damage, +6 to hit, +6 all saves, +4% dodge, +10 PRR
Feats:
* Augment Summoning (+4 all hire/summon ability scores. This is also available as an augment and on several items, does not stack with itself, aug/item versions do not count as pre-reqs for the next feat)
* Improved Augment Summoning (+8 more to all hire/summon ability scores, +100 flat hp, and +10% dodge)
* Scion of Elysium (+25 prr/mrr, +20 melee/ranged power and universal spell power, +100% fortification, +5% chance to ignore all incoming damage)
Epic Destinies
Magus of the Eclipse:
* Great Summoner: +4 to all hire/summon ability scores, more HP (undefined), +100% fortification, +40% move speed
* Share the Light: your hires/summons gain the effects of your Magus of the Eclipse Mantle, and use your cold and negative spell power for its damage.
* Time Stop: 10 seconds of CC and double damage to all enemies hit by it, once every 5 minutes.
Primal Avatar:
* Natural Shielding: +60% max HP, +30 MRR, +4 Reflex Saves, and Evasion to all hires/summons.
* Friends of Nature: various small buffs depending on which animal chosen (only affects those that are active and nearby when you rest at a shrine)
* Shared Mantle: Hires/Summons gain the effects of your Primal Avatar Mantle, using your spell power values for it's damage (can only have on mantle in effect at once, so it's either this one OR the magus one, not both)
* Natures Blessing: +5% insight bonus to max HP after you rest at a shrine (only affects those that are active and nearby when you rest)
Fury of the Wild
* Primal Scream (short term moral bonus to str and con for all allies hit by it. base is +2, but there is an upgrade that doubles it to +4, though investment to get that may be too much)
Items/gear/filigrees
* Diablolists Set (this Robe/Docent + Ring combo drops from the Chronoscope raid. It comes in 3 tiers: heroic, epic, and legendary, and grants all hires/summons a +4/+8/+12 bonus to all ability scores respectively)
* Cry of Battle Filigree Set (requires a sentient weapon or a minor artifact, only available after level 20). Having the 2 piece set bonus grants +4 to all hire/summon ability scores and +20 melee power, and +20 prr/mrr. Having the 3 piece set adds the same values again, thus doubling it to +8 to all scores and +40 to mp/prr/mrr.
Guild Amenities
* Wild Grove: +3 to all hire/summon ability scores
Most of the other hire/summon buffs (inherent/all the time buffs, and short term buffs via abilities/skills/spells, are class specific and come from Bards, Monks, Warlocks, and Druids).
edit: oh yeah, and druid past lives, for +2 all scores each time taken up to 3 times for +6 total. I always forget to mention that one because it's all everyone else always talks about when this topic comes up lol.
edit 2: I should also note that trying to get every possible hire/summon buff on a single character is difficult and makes them are fairly terrible character overall. My charmer is only really "viable" in that one raid. While I can use hires/summons to complete quests, it takes a LOOONG time compared to my other characters. However, so long as you don't push too high in the reaper difficulty, or are very patient, it is possible to do. There are others who have done high reaper with an army of buffed hires, it just takes longer to do. I'm not saying to not do this though, just to manage your expectations.