There's no rule, according to the RAW interpretation, that specifically allows for an IC to attach to a unit that is Infiltrating. The RAW group also claims that the only times you're allowed to attach an IC outside of the movement phase is during normal deployment (putting them on the table top) and with a unit being held in reserve.
Riddle me this:
BRB, page 48 "Special Rules" (emphasis mine)
"For Example, if an independent character without the 'infiltrate' special rule joins a unit of infiltrators during deployment, the unit cannot infiltrate"
The point is, this example specifically allows for ICs to be "joined" to a unit during deployment without putting them on the table top OR in reserves.
This means Shrike can, by a RAW example, join any infantry unit during deployment and, by RAW, confer his special ability on that unit, allowing them to infiltrate.
It's RAW enough for me and my gaming group.
Thoughts?