Hi Roger,
with respect to the Scale tool, yes, it is impossible to have individual parameters of a custom object scaled non-uniformly.
Of course (but I'm sure you are aware, just mentioning for completeness here) such parameters can be scaled non-uniformly via the Attribute manager. Also you could implement handles for such, so the user can access these via the viewport. And the last option could be to implement your own scale tool.
I'm sorry, I know, none of these options is very convenient, but unfortunately I have nothing better to offer.
Cheers,
Andreas