I see that many games that used it would load custom sounds over the default ones.
The MT-32 settings are highly customizable and can make it sound very different from the default sounds, but it's all done with internal wavetables. Unfortunately, there is no way to download wavetable data to the device. It would have been awesome to be able to do so, but it would have made it even more expensive.
The Roland MT-32 could do this, if I am not mistaken.
No. Unfortunately the MT-32 predated DLS.
It did pre-date DLS, but I see that many games that used it would load custom sounds over the default ones. Probably not practical to try to use for anything new, plus the fact that it is not GM compatible, which I hadn't considered when I thought of it.