I have a test application running at test.rsscloud.io on ports 80 and 9876.
The following code works:
curl --location --request POST 'https://brokenriverbooks.com/?rsscloud=notify' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'domain=test.rsscloud.io' \
--data-urlencode 'port=80' \
--data-urlencode 'path=/feedupdated-s8759' \
--data-urlencode 'registerProcedure=' \
--data-urlencode 'protocol=http-post' \
--data-urlencode 'url1=https://brokenriverbooks.com/feed/'
However if you change the port to 9876 it fails. For some reason the rssCloud plugin seems unable to hit ports that are not 80.
I couldn't leave a comment on the reallySimple repo for clouddemo.js (it appears to be locked), so I am leaving a comment here with my report. I was able to run the app on port 443 and successfully register a WordPress.com site and a WordPress.org site using version 0.5.0 of the WordPress RSS Cloud plugin. When I ran my test script on the WordPress.org site, it worked, but I figured out that the WordPress.org site is caching the RSS feed, which explains why feed updates were not seen by FeedLand.
@josephscott - I have looked at this Stack Exchange post for ideas on disabling caching of WordPress feeds, if you have any better ideas, I am interested.
The code for clouddemo.js looks good, I am pleased that it could run on port 443, since I already had an app running on port 80 on my server and did not want to change it.