Well I wouldn't really expect there to be a lot of trust in Luther, for all they know he could just as easily be a loyal Death Eater that is under orders to infiltrate the Ashes and provide information to eliminate them - the Dark Lord did it with the Order (except he didn't know that Snape was really spying on the Death Eaters for the Order, not the other way around) so there's no reason he wouldn't do it with the Ashes. Because of that, I (or Luther, rather) wouldn't expect to be trusted, especially at the beginning. He'd be concerned if they [i]weren't[/i] skeptical, honestly.

Regardless, I've thought of an idea to make your idea work, and could probably combine both that and the letter idea to make a solid solution. Basically Luther learns that there will be a raid on a Muggleborn's house, and he decides to go there beforehand to warn them so they can leave before the raid even happens. There's no evidence that points to Luther (unless they find out about his prior visit, but obviously he would hide this from them so they don't know), and in exchange they agree to pass a message on to Jia. That message being something along the lines of:

[quote]I have important information that the Ashes will want to hear. I will send an owl at dusk with more information. -L[/quote]

This way, even if the Muggleborn was to rat on Luther in hopes that the Death Eaters may grant them pardon in return, they wouldn't have any conclusive evidence to give them, and Luther could play it off as a plot to trick the Ashes into a trap so the Death Eaters could capture them. This would only help to consolidate Luther's loyalty in their eyes, and he could simply hide behind a bush or something and attack them when the inevitable skirmish breaks out, which would also give the Ashes a better reason to trust him as well.

But for simplicity's sake, I'd probably just keep it to the Muggleborn heeding his warning and passing the message on, because the other possibility just makes things overly complicated (although the logic behind it would still apply to why the message is sent in two parts).