Ok, so booked a hotel room, brought plenty of tarps/dust sheets/tape, cleaning supplies, inflatable pool and PVC bedsheets. I'll cancel housekeeping and ask about fire alarm testing.
I've done a few solo messy times in a hotel room, but first time with a partner.

Gunge is a massive turn on for me, but I can also enjoy messy stuff just for the fun of getting messy. It wouldn't do anything for her sexually, but she says it sounds like fun and is enthusiastic about trying it (both sexy and casual fun).

Using MS gunge, Xanthan gum, and VIVI extreme. Possibility of MS instant mud/slime too

Are these fuck/sex friendly?
She's worried about a possible infection/inflammation (hence not using chocolate sauce/desserts).

Which positions work well when we both (very) slippery?

What could be some fun games/ways to get messy?

Any general advice?

" Which positions work well when we both (very) slippery?

What could be some fun games/ways to get messy?

Any general advice?"

This seems sexual in nature so I'm going to be vague to leave your imagination to figure out the rest but here's what works for me since I still got many things to work through on a sexual context.

Fun games to generate mess. (Food fight!) but set it up in a way where it's planned without being obvious. Banter than go all out.
Something like this

Exhibit a
https://m.youtube.com/watch?v=3VZko8PY0A0

Defrost the pies.

Main advice: Focus on the fun, rather than the sexy, to start with. Then see how things develop. A lot will depend on the interaction between you and your partner-in-mess so being able to read how she's feeling / enjoying will be an important part of it.

Good luck, and hope it goes well!

Gunge is totally fuck friendly, as it's many of the same ingredients you find in fuck jelly. Sweets and mud you will find vary greatly from person to person. Some people can get a UTI just thinking about WAM, and others can use an open bottle of Hershey's as a dildo and lube.

As for positions, I don't recommend any of the standing ones

As for making it sexy? That's going to depend greatly on what turns you both on. If your partner is very sensual and like tactile, maybe focus on the slippery with massage rather than just pouring buckets of sticky stuff over her and throwing pies at her face like you're Nolan Ryan.

Consider, what you like then what she does. Then just sort of try to find a way to merge them together. Be excited about trying something new that you love WITH her.

1. Turn up the thermostat. Use warm (but not burning hot) slime.
2. I second no stand-up sex positions. Doggie style is safe and easy to do in a bathtub.
3. While wamming may not be a direct turn-on for your partner, if your partner sees how much they turn you on while wamming that can increase their enjoyment. Be generous with compliments while wamming with your partner.
4. Don't forget aftercare for your partner.

Yeah. Don't overdo it on the first time.

Thank you for all the replies!

Good point on the thermostat- she always turns it down due to hotel rooms being hot and stuffy, but if I'm sat in a pool of gunge waiting for the shower it'll need to be a warm room.
(gunge drips in the bathtub is worse than sheet ice in the winter- no way we can shower share without slipping over)

We'll go very steady to start with- we've actually watched a few porn videos together, looking at what we both like and dislike, we both have some idea of what we'll be doing.
(there's a fantastic amateur-style video of a young couple passionately fucking in green cake batter)
Fun is certainly way way more important than sexy, that's pretty much a general rule for us anyway.

(Also I have some other non-messy plans, in case either of us don't get on with messy stuff)

Don't have to worry about pies this time- but I will expand on this motto for everyone else reading; remove packaging first, then defrost before using.
(avoids damaging the pies and getting in a sticky mess while setting up)

This is really good stuff

In her words: "this stuff is fucking magic!"

My advice for initiating a first-timer is always the same: Tequila

Don't use any foods anyone has allergies too even if not going to be eaten some people have issues w it being on skin for long periods of time / the steam from showering off also can be not so good
Have fun and good luck!!