For future posterity, in hopes of helping other fellow travelers, it *is* possible to run SMACX in Windowed mode, using a 3rd-party tool called DxWnd. This utility hooks the API calls from the app and allows it to be run in a separate window. I am use the GOG version of SMACX. Basic instructions are:
1) Go to DxWnd project page (
https://sourceforge.net/projects/dxwnd/) and download the latest version. (Note: some antivirus tools (like Avast) may false-flag this utility, because it contains API calls that look suspicious. Pay them no mind. This is a long-standing, reputable project. Googling will reveal several instances where this utility has been brought to the attention of AV manufacturers, and they've white-listed it... only to have it eventually fall out of white-list status several years down the road.)
2) Unlike its predecessor, DxWnd-OG (which is what I tried first in my journey, and failed to get running), the new DxWnd is extremely full-featured. It can be a little daunting to try and set up manually, if you don't know what you're doing (which I didn't). Fortunately, you don't
need to know, because DxWnd comes with an extensive list of pre-configured game settings, and one of them is Alpha Centauri!
3) Fire up DxWnd
4) Select "File --> Import"
5) Navigate to the directory you installed DxWnd, and then the Exports folder.
6) Select "Sid Meier's Alpha Centauri (GOG).dxw" (this same config also works with SMACX)
7) Next, make sure that the launcher path is pointed at the correct folder where the GOG SMACX is located.
8) Press the "Try" button in the lower left corner of DxWnd.
9) The game should run.
10) Note that by default, DxWnd launches the host window so that its upper left corner is 50 pixels from top left corner of your screen (X=50, Y=50) and at a size of 800 x 600 pixels. You can manually designate where the window should be spawned. You can also manually designate the window size (1024x768, 1080x810, etc.) Most 4x3 aspect ratio settings will work, but honestly the text rendering is best at 800 x 600. At least on my screen. YMMV.
I hope this is useful information.