What's actually the difference between the usual EV system and Champions' system? Are we losing or gaining 1 point in a stat? If so, it's not worth it to take time developing a different EV system for 1 point of stat, at the very least for the first days Champions is out. Alternatively, just make it so you can run a different amount of EVs in Champions metagames. It's already supported through Custom Games or DVs in gen1/2 on Showdown so it sounds like the best solution.
Essentially how it works is, normally at level 50, the first 4 EVs add one point to the real stat, then every 8 EVs afterwards, which ends up being 32 extra stat points maximum with 252 EVs. Note that this means that the first stat point in each stat is cheaper than the rest. With 508 EVs, this means you get a maximum 252/252/4 which is equal to 65 stat points total if you invest in 3 stats, or 252/244/4/4/4 which is equal to 66 stat points total if you invest in 5 stats. Because EVs are kinda weird at level 50, you get more out of investing in 5 stats, which is partially why VGC EV spreads are so weird. You're encouraged to put 4 EVs into random stats to get more out of the EVs you have. At level 100, it's quite simple, every 4 EVs is an extra stat point, which is why this doesn't matter in OU.
What champions did is make the maximum 66 stat points regardless of how many stats you invest in. For example, investing in 3 stats, you can get 32/32/2 SPs which is equal to 252/252/12 EVs, or 516 total. Investing in 4 stats, you get 32/32/1/1 SPs or 252/252/4/4 EVs, or 512 total. If you invest in 5 stats, nothing changes.
If Showdown wanted to implement this, all they would need to do is make the maximum EVs depend on how many stats you invest in.
Invest in 3 stats = 516 EVs maximum
4 stats = 512 EVs maximum
5 stats = 508 EVs maximum
6 stats = 504 EVs maximum
I do think eventually Showdown will need to fully implement SPs in order to support VGC, but this works in the meantime to fully recreate what's allowed by Champions and what isn't.