Can you use SoftPAC with Node-RED? Yes.
API for SoftPAC (like we do with our PACs)? Our engineers are looking at it.
We have lots of customers running Node-RED and SoftPAC on the same or different PC's.
To install Node-RED on a PC, follow the official guide here; http://nodered.org/docs/getting-started/installation
You can also run groov Server for Windows on the same PC. Its pretty sweet having all three apps on the one bit of hardware, but, and don't get me started, it is a Windows PC in the end, so care and feeding instructions should be strictly followed.
You can of course install the groov and PAC nodes and have them talk at each other all day and all night long.
With the API on SoftPAC, again, because its on a Windows PC, there are some Microsoft challenges for our software team. You need a HTTP(s) listener. A lot of PC's have IIS installed and running, even if you don't think you do, so that can cause port conflicts. The Windows Firewall can cause some issues as well. Then there is the connection between that listener and SoftPAC.
As I said, our team is looking at it.