I'm removing the charcoal canister, but one of the things I want to address is to make sure the small little hard-line tube that runs from the canister back to the fuel tank is not blocked. What is the best way to check that?

Also, I'm think about just venting the main tank at the rear of the car instead of through that pipe. Does anyone know off-hand if there's a connector at the rear fire wall before the line runs into the main tank that I could disconnect and rig up with a breather or such to prevent blockage? Or is it easier to just make sure the line isn't blocked and then put a filter up there at the front in the fender well?

I'll post pictures in a sec...

 

Blowing compressed air through it is a good way; but disconnect the under-car line where it joins the hard line just in front of the LHS rear axle over-axle chassis rail.
On Uk spec cars there is no canister and uner car tube, the hard line mentioned above just vents to air in front of the LHS chassis rail, where it emerges.
Then as long as the Rochester valve in the boot (trunk) behind the tank inside the RHS buttress is working, the tank will vent perfectly.

 

Crawled under the Jag to snap some photos where the small vent line goes into the trunk (boot).

I believe it looks like this...


^^^ Fuel vent hard line join to hose going to boot and main fuel tank ^^^


^^^ Plastic reducer used to join the hard line from the canister to the hose from the main tank ^^^

 

Pirk
The first photo looks like the handbrake cable and the second like the fuel injection return to tank pipe. I could well be wrong as it is hard to make out. If the thing with the white connector is the vent pipe then it will be joined via thata short pipe to the steel vent tube.
If you follow the vent flexible bck from the canister you will find it joins to a small bore steel pipe, about 4 mm in diameter, that appears just near the inside side of the chassis rail in front of the axle cage. On UK spec cars this is where the tank vents to atmosphere.
A few weeks ago someone was looking for the same thing and did post a pic of it. I have no pic of it myself; but someone may post one.

 

Yeah - those pictures don't show the context, so it's hard to tell from that, but that plastic piece is actually a reducer.

Maybe here's a better picture with annotations for those that happen to stumble on this thread.