Is it possible to channel an Ally Spirit in 5e? Would there be any trade-offs to doing so? Alternatively, is it possible to create an Ally Spirit with Hidden Life or the ability to enter into a Spirit Pact?
By RAW, no on all counts.
Channeling (Street Grimoire, p. 148) requires the summoner to forgo the spirit's Manifestation or Materialization power in exchange for treating it as if it had Possession, just with special rules. It dissipates when the services are used up. However, the choice to Channel has to be made at the time of summoning.
An ally spirit is long since summoned, and doesn't have services.
Street Grimoire, p. 201 under "Step 3" says:
Each ally also receives one additional power per point of Force, chosen from any powers of spirits appropriate to the conjurer's tradition.
The point of Channeling is to get the benefits of Possession without requiring a spirit that actually has the power. If the summoner is from a tradition that allows Possession, you can give your ally spirit that power and you don't need Channeling, anyway (though some of the self-control benefits are worth having, even for Possession-tradition conjurers).
Spirit Pact and Hidden Life (SG, p. 205) are exclusive to Free Spirits.