Congrats on getting your 22 AR together, they are tons of fun to shoot. Biggest problem around here is finding 22 LR ammo to buy. I don't see why you can't use a 2 point sling with your lower. I think the slot should be at least 1" which is very common for two point slings. I have bought several Blackhawk 1" wide two point slings which have good quality and generally sell for less than $15. All you need is the quick release to thread onto one end of the sling and thread the other end through the slot on your lower. I like the Blackhawk sling because it has plenty of length for just about any long gun and are inexpensive.
The sling pictured with the CAV-15 polymer lower is designed to have the sling attach to the side of the stock to enhance carrying the rifle in the assault/patrol position. This basically means the rifle will ride more or less horizontal in front of your torso, depending on how the sling is adjusted it is easy to have the rifle hang horizontal, muzzle down or muzzle up. The weapon can be easily carried In this position for long periods of time and allows the user to control the rifle with one hand on the pistol grip while opening doors. Most any 3 point tactical sling can be configured in this fashion. Specter Slings makes a sling for the A-1, A-2 butt stock which uses the same strap to attach to the butt. I know Brownell's sells several version of Specter Slings.
Here is a link to Specter's site, they have numerous products of high quality, I used the SOP 3 point sling on my 16" AR-15 duty rifle. The front mounting hardware can easily be adapted to use a quick release swivel.
https://www.spectergear.com/ProductDetails.asp?ProductCode=132