Stiff-up your brush: paint porn

I think you'll struggle to get anything to adhere robustly to powdercoat.
Powdercoat is usually polyurethane which has form for being difficult to get stuff to stick to it.

same thought as above, could you DIY paint the whole thing then send it to be clear powdercoated?

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Sounds like what @TvH did with his jellybean Salsa

Powder contains no solvent so you'll struggle to have anything bond well to it.

We frequently do this type of finish with customers at Quinntessential.

We'd use an epoxy primer, tougher, higher building and far stronger than an etch priming process. Then we can base coat in your preferred colour, next clear coat and then key it to 600 grit for your mechanical bond.

You could then go ham with your preferred medium - POSCA pens would be our recommendation.

Once you're done, bring it back, we'll clear it again and flatten and polish.

Yes, it won't be as robust as powder but it will hold up well if cared for.

If you were to go the powder route, you could just use nail polishes.
They're enamel and they'll harden without too much encouragement.

If you have nice paintwork that you'd like to take care of, you can't do better than Chris at cyclewrap UK.

Good luck.

Very late to the party here but I did this with my YT Capra, had the frame powdercoated, went at it with Posca paint pens and then some 2k clear from auto factors on top. It's help up remarkably well and that's as a bike that gets off-road abuse and rough washings so I'd feel quite confident in doing it on a road bike and having good longevity...

...after painting

...a recent shot four years later