|The Kwisatz Haderach|
Could an Unseen Servant hold a banner or Standard such as a Banner of the Ancient Kings or Standard of the Celetial Heir to provide the Morale bonus to allies?
Hold one, yes. Provide benefits, depends on the item.
When carried into battle, a banner of the ancient kings confers several benefits. As long as the longspear or pole to which the banner is attached is firmly wielded in two hands, its carrier gains a +4 circumstance bonus on Initiative checks. In addition, when so wielded, it grants the wielder and all allies within 30 feet a +2 resistance bonus on all saving throws against mind-affecting effects.
Unseen servants don't have hands, so technically they can't wield it properly in the first place. Some GMs will overlook that, though. More troubling is the fact that an unseen servant, as a mindless force, probably doesn't get to have "allies" even if its caster does.
A command word causes this bright white flag to change its emblem to match the deity worshiped by the bearer. Creatures loyal to that deity gain...
An unseen servant definitely does not worship any deity, nor can they speak command words.
This issue has come up before. The traditional approach to flags and standards is to mount them on a longspear. This is convenient, as the longspear is an excellent weapon whose use is unhindered by a bit of cloth. There are even several examples of this in the ancient Bayeux Tapestry. So that's pretty much a solved issue: if you want to display a flag or standard, carry a longspear and mount it on that.