What about launching SAP GUI for Windows from CRM WebUI? This actually available in the standard from October 2013 (along with zapp_integrator project and another approach Development and Extension Guide - Launch SAP® GUI Transaction from WebClient UI ). Take a look at notes:
1870102 - SAP GUI Transactions from CRM WebClient
1890755 - SAP GUI Transactions from WebClient UI
1840776 - Launch SAP GUI Transaction from WebClient UI does not work as expected
The idea to convert GUI windows into BSP pages, as mentioned above, is good in general. But very complex from developing perspective, IMHO. It easier to switch ERP to web-based UI instead 