That's very strange. Just last week I used PayPal for a refurbished 70-200 RF with no problem.
BTW, for anyone following this thread, after I called Canon the lens did indeed ship and arrived today, in the usual pristine condition that I'm used to with refurbs. I will always suspect that a substantial portion of their refurbs are actually new items where the packaging was damaged during shipment. Of course I have no evidence of that.
Update:
After my wife noticed my Canon Store shopping cart was empty, she added one of the items I intended to purchase. When I've placed items in the cart, I have always started with the 600mm lens. She started with a different item, and the PayPal option was there!
After she informed me, I added the backordered 600mm lens. The PayPal option disappeared.
I removed it and the PayPal option reappeared.
So it turns out the issue all along was that adding a backordered item retracts a "pay immediately" option. When this all started, the 600mm was listed in stock, and the PayPal option was available, therefore it did not occur to me there were stock limitations for this option.
It apparently did not occur to the various Canon representatives I spoke with either.
Now I've ordered and received the two other items, and have the 600mm in my cart. I'll check daily (with fingers crossed) to see when one arrives.