I'm using a S-Series PBX, and wanted to connect the MOH to an analog input, but I know the S-Series doesn't have the option for an External MOH, so instead I figured I use a SIP client connected to the input and set the MOH to be that client, as either an extension or conference room. Is there any way to do that?
In short, no.
Without going into all the details, the scenario you have painted is one in which you are transferring the call to another extension. Once done, you have essentially relinquished control to the other extension and they are now the owner of the call. There is no way to retrieve the call.
However, more importantly, is that there is no way to point an extension as being the MOH source.
Many IP-PBX manufacturers have elected not to provide for analog streaming because:
1. It increases the cost of the device (analog to digital processor)
2, The analog stream must be transcoded into the needed codec(s) thereby adding processor utilization, which may either degrade system performance as more calls go on-hold or require more robust hardware (more cost) to accommodate
3. Copyright infringement - not that everyone does, but many will use a radio station feed or other material and use that for the MOH, most unknowing anything about copyrights and royalties.
However, I too would like to see an input where I could use it. The biggest complaint is that should a call be placed on-hold several times during the course of a call, the recording starts over.