It is a known issue, but there is no estimation for when this could be implemented (hint: It has been on the list of known limitations for about almost years).

At least with Edirol Orchestral you actually can use several instruments. Just change the Midi Channel number in the Instrument-Tabs.
BTW The only downside of that plugin is that you have to manually load and save its preference because it isn't saved with the mod. (but I guess that's a VSTI-related problem, as I haven't experienced it with other plugins...)

The point isn't that the author wants a multitimbral VSTi, the point is that many multitimbral VSTis can output their individual instruments on different "pins", i.e. they don't get all mixed into the same buffer. If OpenMPT could make use of that, you can e.g. put a reverb on one output and leave the other output untouched.

Regarding Edirol Orchestral, this is already written down in the plugin FAQ - I suppose the plugin is meant to mimic a classic hardware synth with no "total recall" and thus doesn't have this feature.