what you could already do is to include the IPs of the synthetic (web check) locations for your internal applications. You can find the IPs in Dynatrace Web UI during creation & in the settings of a web check.
As @Kristof R. stated we are also currently working on HTTP monitors which will be capable of monitoring the availability of internal applications from within your infrastructure. We plan to start a Beta for this feature beginning of next year.
Kind regards, Philipp