My username is more hope than truth. My easy going lovely wife will participate in sploshing , on occasion.
Often offer to do it, but come the event, when all is ready, can't bring herself to do it! I don't blame her, it's a big ask. I'm wondering how other couples are doing?
Mutual wamlove, one sided desire,. How does the other half behave?

Might just be nerves/inhibition, if she offers to do it then changes her mind. Its a big difference between talking about it and seeing the full setup.
If you've gone as far as getting set up (and it is just shyness) then try asking her to get you messy. She stays clean but still is involved as a way to break the ice.

If it's more than shyness you both need to talk it out, and not push it any more until you're both fully comfortable and understand what the other wants.

My partner really hates the cleanup and getting it out of her hair, but enjoys being the one dishing out the punishment. If were both getting messy she usually wears a swimming cap. It doesn't look sexy but it keeps her hair tidy.

So, I think the fact she's offering is a real positive - even if she does get cold feet. I suspect there may be so something specific stopping her, and if you haven't already do try to explore that with her. We had a similar dynamic - when I told my wife about my fetish she was willing to give it a go to please me. After the first couple of times though it emerged she wasn't keen on a couple of things - specifically the feeling of sticky substances on her skin, and having large quantities of anything poured over her head - she feels claustrophobic/overwhelmed. Pouring custard over her head was amazing, but much less amazing once I knew she didn't like it, so we worked round it and now when she gets messy it tends to be smaller scale and no sticky substances. We've also incorporated a lot more of her getting me messy/dominating me, which she really does enjoy. The strange thing is it was something I really had no urge to try until we had to improvise, but now I love it - most of all because its happening with her. I'm sure if you keep communicating openly with each other you can get to a place that works for you both.

It took us a while to figure out the specific situations in which my wife would feel comfortable with it.

Sitting in the bath and waiting to get slopped? Not something she enjoys at all.

Taking a pie in the face during a passionate sex session? She actually initiates that more than me

I try to put myself in her position, and be like "If WAM didn't turn me on, would I want my partner to sit me down and cover me in stuff?" Not really, it'd be pretty annoying and nervewracking I think!

Also, if it's a time where she's not feeling comfortable with the idea, I often make the suggestion that she just does it to me instead.

But yeah, it's worth trying to figure out what works best for you two, and maybe find a compromise that she feels more at ease with. Don't push it though. Best of luck

Definitely one-sided.

She's more than happy to wear a dress in the shower on occasion, but nothing more than that. Which, honestly, is more than fine with me. We've tried to do messy stuff before but it was just awkward. So I have full reign to buy scenes and stuff as long as she knows about it.

That's a really good point. Because, even now, the premise of many wam scenes is basically 'girl (or guy) sits there and takes it', it's easy to forget how unnatural that scenario would be to a non-wammer.