It seems like there would be other parameters that should probably be LTR as well, such as callback?

Yeah, quite possibly.

And you probably meant LTR :)

I did! Already fixed :)

There's a decent chance that you'll have to add either a new value for ApiBase::PARAM_TYPE or a new constant to indicate LTR directionality; which one depends on which fields, exactly, this might apply to (e.g. URL fields would make sense as a new "url" type, while something like the callback parameter probably wouldn't make sense as a new type unless it's extremely generic like "ltrstring").

Then you'd need to update the return values from getAllowedParams() for all the applicable API modules, and the ApiSandbox JS to apply the correct directionality to the widgets.