Sustain loop on a sample but have the marker jump to the end point of the loop

[chakastab]

Is it possible to do this in OpenMPT? I was thinking of having a sample end early before it reaches the loop region, making it sound like a note that's being played naturally, like you would on a keyboard. Or do I just use the envelope for that....

[Saga Musix]

Sample sustain loops always play in full before they are left. If you just want the sample to fade out somewhat naturally, you could use the instrument fadeout property, or indeed set up a volume envelope. Unlike samples, envelopes do have an additional release note in the MPTM format, which OpenMPT will directly jump to as soon as the note is released, so it doesn't need to finish playing a sustain loop in full.
If you do want to use the end of the sample past the sustain loop immediately, the easiest way would probably be to just copy the tail of the sample to a different sample slot and trigger it as a separate note.