Robot Manager and WFI is only compatible with versions Pega Robotics 8.0.1094 and higher. You would have to push the RuntimeConfig.xml with the WFI settings manually and only connect with Robot Manager (not deployment portal). The client would still need an OMC tenant and setup on our side, but all authentication would be done through RM. The tenant (deployment portal side) would only be used for the admin to check and generate reports, etc.
In short, yes. You will be ok with that scenario with a few adjustments.