I seem to recall that a TSU9400 IR transmitter is less advanced than that of a TSU9600. Do you happen to have a 9600?
I just reviewed the PDF, yes, you can do this but be aware that Envy requires a heartbeat every 20 seconds. This means that if your unit is left undocked and script stops executing, you will have the socket closed so you should be prepared to use onSleep/onWake notifications to create new socket each time the unit wakes up.
When you connect, Envy will send a welcome text such as this, which you don’t need to reply to: WELCOME to Envy v1.0.1.0 ⏎ It is recommended that you keep the connection open. Please send a heartbeat signal every 20 seconds. If Envy doesn't receive any message from you within 1 minute, it will close the connection. The heartbeat signal very simply looks like: Heartbeat ⏎ Envy will reply with "OK ⏎" to the heartbeat. If you prefer, you can also open a connection, send a command and then directly close the connection again. But you'll not get regular notifications this way, of course.
All this being said, I'd recommend installing a raspberry pi or some other type of proxy that could be used between the Envy and the Pronto. The proxy would manage heartbeats, etc. Then, all you have to do on the Pronto is to send/receive the commands/responses/events you want.
The protocol specifies a CR message framing and this is exactly the type of format I have for my DenonProxy which I wrote using NodeJS.
Also note that you may encounter issues with File Uploads/downloads as the response will be terminated with a CR and then there is a specific number of bytes sent as base64 encoding.
If you don't want to do file uploads/downloads from the pronto which I would think you would not want to do, then you should be OK.
Contact me via email on my profile if you are interested in some Proxy Source Code in NodeJS (Javascript using V8 engine).
