I mean... There's no reason you couldn't, but I don't think it would work the way you're thinking it would work; Enhance Signature Spell doesn't affect the level of the Spell, only the damage/healing, and United Blow grants damage based on the Spell level, so ESS-ing it wouldn't affect the damage from United Blow at all.
The only time the effective level of a Spell changes as a result of effects like ESS and Potency is when they're being Counterspelled or absorbed into a Spell Store, and then only because the recipient has no way to know whether the damage has been increased or not (and those times it's obvious that it has, such as any number higher than 45, can't be Counterspelled or Stored, anyway, because there's no such thing as a Spell level higher than 9).
United blow uses similar wording to combined strike, and per previous rulings, as soon as you use a spell with combined strike/united blow, the spell is no longer being cast from memory, and isn't valid for things like enhance signature spell, powerful meditation, etc.
Say I'm a spellsword with a staff. I have 9th level Signature Spells memorized, Combined Strike, Foundation/Magistarium blow, Potency, Magic Evocation, Oak of the Archmage and Enhanced signature spell High Magic. Can I combine all these? Celestial Spellsword: 100 Spell Strike Flame (Oak OR...