My (likely flawed) understanding is that you guys can custody funds but you can't custody+transmit them without some crazy expensive license, right? Or are both off limits?

With a chaumian ecash system, it doesn't seem like you would be doing the transmitting. The transmitting happens peer to peer, out of band. Ecash has essentially the same properties as a physical gift card; whoever holds it can give it to someone else. The issuer can choose to honor it regardless of who brings it back to them.

Arguably, you wouldn't be doing any custodying either, especially if you are upfront about the fact that you won't guarantee that the ecash will be redeemable for sats. Then you aren't holding anyone's sats - because there is no agreement that those sats belong to anyone but you, until the moment you decide to allow a redemption.

You might look towards Roblox Corporation's "developer exchange" program as an example to see how they have carefully defined "Robux" as something which can "sometimes" be exchanged for real money, but also Roblox is under no obligation honor the exchange - they reserve the right to renege at any time, change the exchange rate, etc. In practice, they readily exchange it for USD at a stable rate always. It appears they may have crafted this language specifically to avoid being classified as a custodian of any kind.

In theory, I think you could apply something like this to Cowboy Credits, and in practice, most of the time, choose to allow people to redeem them for sats. Even better if users handle the transmission themselves in a peer to peer ecash style. Basically I'm saying, issue an ecash-style token which is, at your discretion only, with no guarantees, redeemable for sats.

But I assume you've probably already talked with some very smart lawyers who have explained why it's not actually equivalent to a gift card. But maybe have them look into what Roblox is doing with the developer exchange if they aren't familiar.