Based on the digging around I've done over the last couple of days, I can't say that using IBM TTS is either legal, nor illegal, as the licensing terms that are present are just incredibly sparse, which is definitely atypical.  That being said, and the fact that the software is abandonware and is available on an open FTP server, the only conclusion I can draw is that IBM is making this available to any and all who wish to use it.  Companies know how to protect the assets they wish to protect, and even "free stuff" generally has far more extensive licensing information than is available in the folders on that FTP server.

So, based on information provided to me by Mr. Kennedy (thanks), and to close this topic out, for those who want to use IBM TTS, under the presumption that it's legal to do so:

1. Someone named Jake Gross has packaged all the viavoice tts languages into one installer, available here: https://grossgang.com/tts/old_software_synths/viavoice6732l.exe 

2. The Add-On for IBM TTS, sans voices, is available on GitHub: IBMTTS driver, Add-on for NVDA
     There is a download link for the Add-On present on that page.  Search for "download in this link" on the page.

3. Once you have installed both, IBM TTS will not show up until you tell the add-on where to find the ibmeci.dll file. You do that in your NVDA settings under IBM-tts. And the ibmeci.dll is located in C:\Program Files (x86)\viavoice … only after IBM TTS is installed, so make sure you have installed both it and the add-on first.

I have not tried any of the above, by the way.

And with this, I am locking this topic.  Out of support software is not supposed to be discussed on the NVDA Group, period.  If folks want to try all this out and discuss it further, it should occur either on the Chat Subgroup, via private messages, or via private email.  IBM TTS is no longer a valid topic for discussion on the NVDA group not secondary to issues of legality, but because it is long, long out of support.
--

Brian - Windows 10, 64-Bit, Version 21H1, Build 19043  

I do not understand why some seek to separate a person from their actions.  The self is composed of an individual’s thoughts, actions, and expression, which are contained in and actuated by the body.  What you do and say is the clearest indicator of who you are.

      ~ Brian Vogel