Here's a couple of options:
1. Buy one of the BPL fly rod cases that's 36" long and cut it down to fit the rod you have or will get. BPL has these rod cases in two diameters: 1.5" and 2.0"
2. Use the 24" BPL rod case without putting the cap on one end, so that your rod (in its sock) extends beyond the end of the rod case a few inches (and be sure to keep the open end up when secured to your pack)
3. As for your main question, don't know of a 5 piece fly rod made by either St. Croix or G. Loomis (check their websites for the answer to that question).
If those two companies don't make a 5 piece version of what you want, and if 5 piece is what you have to have, then you might check rods made by the following companies which have (or had) 5 piece fly rods. Redington made a 6 piece, too, but don't know if it still does.
Winston (the LT 5 piece)
Cabelas (the Stowaway 5 piece)
Redington (the Wayfarer, in both 5 and 6 pieces)
The Redington Wayfarer may not be made anymore, since I believe the company was bought by Sage (check websites). In any event, the Wayfarer rod can probably be found new or used on eBay, as well as rods by the other two outfits.
Winston is very expensive; the other manufacturers have much lower prices, but make nice fly rods nevertheless.
From my limited experience, one fly rod that breaks down to a very short length is a Redington Wayfarer in a seven foot length with 6 pieces, breaking down to 16".