Hey,
So for years I've wanted this functionality as well (to set up remote outposts that auto-request delivery of missing requests). I've used Ghost Scanner at various times in small networks when the performance impacts were acceptable (many to the original author of Ghost Scanner and all those maintaining various updated forks of the mod!)
In the distant past, I had also looked at an alternative approach - scanning the 'missing construction requests' alert output and converting that to signals. But at the time the alert API in Lua was so limited I gave up (this was well before the 1.0 release, etc.)
Seeing Factorio 2.0's improved alerting behavior in the UI made me go re-check the Lua API and I was pleasantly surprised to see a nice full and well-rounded API! But, it seems that actually the API was improved well *before* 2.0. Eventually this led me to finally stumble on a mod that already exists doing exactly what I was thinking (albeit, via a MUCH more complicated algorithm than I was even thinking to minimize UPS impacts):
https://mods.factorio.com/mod/AlertScanner
I had missed it so far because it was only available for 1.1 not 2.0. And it was 'new enough' that I missed it back in my older Factorio days.
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I pinged the mod author to ask some questions about why he did various things in the logic because I was interested in trying to either port it to 2.0 or make a similar mod on my own. But, as a result, the author took the initiative to update the mod for 2.0. Super service!! Thanks @Platinius <3
I still have to seriously try it out. I did at least verify that it seems to work and output signals. It has the downside of clearing missing construction alerts for "monitored networks" but that maybe is acceptable?
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Just thought I'd post this for reference. Both approaches (Ghost Scanner and Alert Scanner) are of course valid and have various upsides and downsides. But I felt I was monitoring this general problem / thread / solutions pretty carefully and yet I never spotted the Alert Scanner mod till today. So I figure maybe y'all would be in a similar boat, too.