Basic series recording was always possible. First create a timer for a single recording and then edit the timer and play with the "Day" field settings. You will notice that you can change it to "__-__-__-__-__-Sa-Su", "Mo-Tu-We-Th-Fr-Sa-Su" etc.
In this way it is already possible to create a single timer for series that air on the same channel and the same time, but on 1 or more weekdays.
As you will notice, this is far from user friendly and it won't give you all the options that MediaPortal's TV Server support.
The challenge with the XBMC approach is that different backends support different options for series recording, so there is no way to have one single "Series recording" dialog that will fit every current supported backend.
Luckily, XBMC allows me to show custom dialogs, and I'm now in the progress of adding a custom "Series Recording" dialog to the Gotham version of the MediaPortal PVR addon.
A quick preview:
This dialog is not yet in the official XBMC Gotham alpha builds and works currently only for the Confluence skin, but it it at least a start