The body on my '87 244 is in pretty good shape except for the rocker panels under the doors. I'd like to get rid of the big holes rusted out in that rounded sheetmetal structure that begins at the bottom of the car and curves around and up to the base of the door frame. Are these easily removable, replaceable or is there an alternative (plastic pipe, steel pipe, etc.)?

Depending on how bad it is, you could just pop-rivet in some pieces of galvanized sheet metal. You can pick it up at any heating shop.

Alternatively you could source out some repro rockers and have them welded in after hacking out the old ones.

If the rocker is still generally solid a few w/holes, you can cut back to good sheet metal and then fabricated some patches salvaged from spare door/hood/etc sheet. I once got some very nice, brand new sheet metal panels from a custom van shop, pieces they'd cut out for window/sunroof installs. They gave me several large pieces, enough to do my repairs, at no cost.
Shape the patches and braze them in, grind the welds down, body fill to blend, and re-top finish. I use a cold galvanize spray as a weld through primer on the back side of the patch and refinish the front with Por 15 after it's in place then, body fill, sealer and top coat. Patches I've done on a couple of my Bricks have stood up to 5-6 years of winter salt/slush without any sign of further corrosion. Hope this helps.

That panel can be bought as a spare part. I changed one about 5 years ago and I must admit that it took a few hours...
You have to cut the old one off and weld the new one on.

Where did you buy the panels?

There is a Volvo green manual that gets into the details of replacing the rocker panels. Probably worth getting if your going to do the job.