Can't say for sure here either, but...
I had an '07 FLTR with a stock front fender (not lowered) and a 21" wheel running a Metzeler 120/70-21. It was a close call with the standard height fender. You couldn't see the tire under the fender.
I tried replacing the stock fender with a Fat Boy fender with the fender mount blocks that Harley makes for that fender on a touring bike. Had a rude awakening when, after having had the fender painted, I couldn't get the wheel back on with the fender on th bike.
Oh, and for the record... 16" and 18" wheels are pretty much the same height. While the 18" wheel is larger, the rubber is not as tall on the 18" tire. A 21" tire is about 1.75" taller than an 18" tire. Again, the rubber on the 21" tire is not as tall as the 18" tire. That means you need an inch or more of clearance inside the fender on the 18" to be able to get the 21" on comfortably.
My guess would be that the 21" is probably not going to fit. If it does, it's going to be a close call.
You're doing the right thing by asking around.
Oh, and don't take the dealer's word for it. The dealer swore I couldn't run a 150 tire on my 18" rear wheel on my '07 FLTR. Said it would rub. Well, bullchit. I ran 150's for 10K miles with no problems.