Sorry, this is not possible with Rack Instruments.
I would recommend you to use Track Instruments. In fact, there is no reason to use Rack Instrumnets, in these days. Track Instrument can also be a multi-timbral, and you can use multiple-outputs (which were the domain of Rack Instruments only in early days).
You can use an Instrument Track, and then you can Add a MIDI track, and route the MIDI track to the Instrument Track. Then, you can use a MIDI Send on this MIDI Track. So the result is the same, again.
I mean the same in sense, you can use MIDI Sends with the Instrument Track, but you can also use all advantages of Instrument Track (like export/Import Track Archive) at the same time.