I have JMS running on a Joomla 3.4.5 installation. Is it possible to use reCaptcha2 in a JMS site? As you probably know, each domain must have it's own reCaptcha keys. In my installation, each slave site has its own database, so I am thinking that I can just leave the master site keys blank, and add each slave site's keys through its own administrator area. Will that work?

It would be nice to know in advance. Though I can't see why trying it on my own would break anything. If I don't hear back from you soon, I'll try it an will post the result here.

For others, you might elaborate in your reply whether reCapthcha can be configured in a JMS installation with 1 shared database.

Thanks!

reCaptcha is provided in standard with Joomla.
Inside the reCaptcha plugin, you can select the version 1 or 2.

The parameters of the plugin are specific to each slave site.
So, YES, you should be able to setup each slave site plugin with their specific keys.

The sharing of the plugin parameters is not possible.
The parameters of the extensions (component, module, plugins, ...) are located in the "#__extensions" table and this is the table that defined all the extensions installed and enabled/disabled.

There is just some exception with some plugin that does not save their parameters into the core joomla table (#__extensions).
Some rare extension may save their settings into a specific table or file on the disk. So in this specific case only they can be shared.
This is not the case of the reCaptcha that use a standard joomla parameter saving and not a specific one.

Merci!
Yes, I was able to set up each site with reCaptcha2 using individual keys in each slave site. No problems.