Yes, may be possible, but horribly inefficient! The labels are interactive; they change color on hover - so the renderimage has to be created every frame! And every text has different size. One little renderImage for each label... that's slow... and even worse: it's not supported on every graphics card!
Well, it doesn't have to be implemented using the worst algorithm ever
The highlighted version can be generated together with the normal version at init, and then switched using SetSubRect.
You don't have to use one RenderImage per label, use a single one and store the resulting sf::Images. You can even group several labels into big images.
Support for render images is really really good, I don't even know if you can still find GPUs that don't support them.
I know it's not a clean solution, it requires some extra work on your side, but I have nothing better to offer (at least after spending a few minutes to think about your problem).
The new drawable system will hopefully allow cleaner solutions for this kind of problem.
Why isn't there a FlipX() method for texts? Would be really useful for reflections, too...
Because it's complicated to implement, and it impacts consistency of other functions.