The only question would be whether the freebore on the rifle is too long to load the 80s optimally. If you have a chamber cut with PTG's 223 Rem ISSF reamer (0.169" fb), you can still load Berger's 80.5 Fullbore bullet with sufficient bearing surface seated in the neck to work. If the chamber has 0.200"+ freebore, maybe not so well. If you have/use a Hornady OAL gauge, it's easy to make an estimate by simply seating the bullet out to just "touching" the lands and see where it sits in the neck.
In the event the freebore is simply too long for the bullet of choice (i.e. not enough shank in the neck at "touching"), the alternative is to jump them a greater distance than you might normally. It can still be possible to tune a load using seating depth with a bullet that is shorter than would be optimal for a given freebore chamber, but you may have to carry out the seating depth test window starting farther away from the lands to maintain sufficient bullet shank in the neck and test over a pretty wide window.
As has been mentioned, there are other options such as the Sierra 90 SMK or Hornady's 88 ELDM, if you can find some. The bearing surface length of the 90 SMK is about .010" longer than the 90 VLD, so the amount of shank in the neck at a given seating depth will be pretty close between the two. The issue with the 88 ELDM is that it has a much longer bearing surface than either of the two 90 gr bullets. Even in a .223 Rem with 0.220" freebore, they are seated pretty far below the neck/shoulder junction. So in a rifle with 0.169" freebore, they would seated way down in the case. That does not mean they couldn't be used, just that you'd have to be much more cognizant about pressure, and you might end up by necessity with a load that was slower than desired due to the potential pressure issues and the velocity of the uppermost node you could hit at safe operating pressure. Killing the primer pockets of the brass in a single firing is not a good way to go.
I have shot the 80.5s numerous times at 300/600 yd. I also know a couple F-TR shooters at my local club matches that use the 80 VLDs at 600 yd. At 600 yd it can certainly be done, but if there is much wind, your wind calls will have to be a LOT better in order to be competitive. Even when loaded to somewhere in the 2900-3000-ish fps velocity range, the 80s are giving up a lot to the 90 VLDs/.308 shooters, not just a little bit. The difference in expected wind deflection would be on the order of 0.6 MOA or more. However, the benefits to going this route would be that you can still participate and have fun, and your wind reading skills will likely improve. Your plan of using the 80s at 300 yd, thereby saving the 90s for 600 yd work is a good one. I used to do exactly that in local 300 and 600 yd matches. You still have to be on your toes with the 80s if it's windy, even at 300 yd. However, the predicted difference in wind deflection between the two choices is obviously much less, maybe </= 0.25 MOA. It's doable.
I wouldn't be surprised of we see new production runs of the 80-95 gr .224 bullets from companies such as Berger and Sierra, with bullets becoming available sometime from mid-summer to fall. This is solely a guess on my part (i.e. I have no inside information whatsoever), but it seems reasonable based on announcements made by these companies regarding bullet production about one year ago. Keep your fingers crossed and maybe these bullets will show up before the shooting season ends.
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