I have some 1/2" PEX-AL-PEX left over and thinking about using it as conduit for ~20' ethernet or phone cable. Pipe is appropriately orange.
Will the aluminium layer provide RF shielding? (assuming I manage to ground it)
Is this a bad idea?
Because this is an indoor run, I'm not entirely sure why conduit would be helpful, except in eventually pulling more cables, as you mention. That said, I pulled CAT6 through 3/4" PEX, and that was difficult. If you have 1/2", you're not likely going to be able to pull it through at all if it's terminated, and you probably can't get any more cables in there afterward.
Overall, I don't see a point to it.
If you wanted to do it, you could. You could probably use a PG fitting to connect it to boxes on either end. Keep pull tape in the PEX so you can pull more easily in the future. It helps to have twice the length of the run so you can hold it on either end, attach your cable, and pull without losing the pull tape on either end.
Meh. You might get some benefit in the shielding department but it's a lot of hassle for not a lot of gain. Phone lines don't need much, if any, shielding. Ethernet is internally shielded to meet the needs of the signal inside (i.e. this little bit of conduit won't bump you from Cat5e to Cat6).
The only way I would do this is if you have an open wall and want to be able to run wires later. But 1/2" PEX will hold maybe one wire at best. Still not much to be gained there either. If you want conduit, PVC conduit is stupid cheap.
If you are located in the US and this is going to be subject to inspection according to the National Electrical Code, then you need to use a listed raceway. Pex tubing or any other plumbing pipe is not listed for use as an electrical raceway.
If you just want some physical protection for the Cat 5 cable then PVC electrical conduit will perform nicely. Save your scrap pex for a plumbing project.
No separate RF shielding is needed for typical installations with twisted pair Ethernet (e.g., CAT 5) cabling. Phone cable also doesn't generally need RF shielding. I would actually recommend using the same cabling (but separate physical cables) for phone & Ethernet cabling. Phones don't need the top quality of CAT 5 (CAT 3 is more than adequate) but for 20' it is easier just to do everything the same way - just the connectors/jacks on the ends are different.