Thanks to all for your help!

The solution I ultimately went with was to use iisexpress-proxy to set my IIS Express's local port to one which was (temporarily) remotely accessible. This is the same general solution as Dev Tunnels and ngrok, which likely also would have worked--I just arrived at a working solution with this solution first.

The particulars for me required:

• installing node.js (https://nodejs.org/) - Run the Windows installer
• Make sure the installed NodeJs folder is in my Paths System Variable: C:\Program Files\nodejs\
• Install iisexpress-proxy as a global npm module with npm install -g iisexpress-proxy
• Activate the proxy by opening a command line and typing iisexpress-proxy localPort to proxyPort (e.g. iisexpress-proxy 54449 to 3000 to map my IIS Express local development port 54449 to 3000)
• Have my Android app access the testing url at LocalHostIP:3000/myurl?param1=x&param2=y

Only real downside to this approach I see is the need to remember to create the proxy every time I test this way (likely create a one-click batch file to do the iisexpress-proxy command). This seems like a mild hassle, but I'll admit to being slightly nervous about any alternatives which would automatically create a link to my dev machine which might be visible to the outside world. Please advise if I'm being overly nervous about this, or if you have a better development path for such work. In any case, thanks again for all your help!