S4 method implementation for S3 class

There is formally no such thing as an S3 class. Just S3 objects with a
class attribute. You could extend a base class with an S4 class, like
setClass("IntegerWithUnits", slots=c(units="Units"),
contains="integer"). Sure, that's a disruptive change, but it would be
in the right direction.

Extending base classes is always a risky proposition, as you've
discovered. Ideally you would override every transformation in order
to adjust or carry over the representation accordingly. The problem is
that there's a huge amount of transformations available for base
classes, mostly not encapsulated by generics.

Other possibilities include:
- Convincing someone to make %*% an internal S3 generic
- Promoting %*% to an R-level S3 generic, which would only work with
code that sees your namespace

Hopefully others have better ideas,
Michael

Thanks for your kind help and suggestions, Michael. I think I'll take
the last option as a workaround, at least until I evaluate the
advantages and drawbacks of a complete redesign.

I?aki