I do not think you need to use the Web-API. You do need to interrogate the database which will hold in one table the list of connected users, and in another table the hostname of each user (assuming you put it there).
The problem is then to use the H-P agent (or any other agent, for that matter), to run on each of those hosts the banner application.
1) run a rule that passes as parameters any information you wish to include in the banner message.
2) that rule runs an external program or script.
3) the external program or script: a) runs a query of the database, which it either writes to a file, or runs a PL/SQL script (if you are using Oracle), selecting all the currently connected users, together with their PCs' hostnames; b) for each row returned, or each line in the output file, activate the agent, which runs the banner program on the PC.
I suppose that there must be documentation somewhere on how to access the H-P agent, but I do not know exactly where.