I´m not sure to understand your question... If what you are developing is a propietary (your own) serial protocol to execute some tasks from different platforms (with different programming languages), all talking your protocol, and what you want to do is to plug a serial cable from/to PC-device and send from the PC serial commands of your protocol to the device (could be another PC) to test your software, yes, there are plenty of software to send serial data:
With some limitations on the free versión: http://www.commfront.com/commfront-downloads.htm
A full free software from Extron (you can send a script file): http://www.extron.com/product/software.aspx?id=dataviewer&s=5
There are many more, specially if want to waste your money, but those are the ones i´ve worked with for some years and works great.