I always thought for most applications on RISC OS, <Enter> takes you to the next field, except the very last field, when it performs the default action. There's even the validation string "Ktar" which I thought was for this ( [link] ).
Is this what you're talking about? If so, I'm surprised this isn't how it is in the style guide!
Curiously, one of the difficulties I have with ThumbCat is that you can't move between writable icons in this way. It's a really minor thing, but it looks like I was wrong to think it ought to do this anyway!