Did some further testing with the smart:limit in trial mode.
The constant gain functionality is basically linked input and output and this indeed does not work properly; as soon as gain reduction takes place the output volume is lower and lower.

So, forget about my feature request. I thought Sonible was smart...

Smart:limit sounds very nice though, very transparent, but TDR Limiter 6 GE can sound exactly the same if you leave the brickwall functionality off in the limiter. Or when using the clipper in Limiter 6 GE, be sure to use a soft knee. Same results, just different user interface.