You can edit the below registry settings but I don’t recommend it. The Stream Buffer Engine v2 used by Media Center does not take network latency or packet loss into consideration as it is optimized for a high IO stream.
[HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionMedia CenterServiceRecording]
RecordPath
After changing the value restart or run the UpdateRecordPath scheduled task that sets the proper permissions on the folder.
Depending on your NAS (if it supports Home Group) you’ll also need to change the ehRecvr and ehSched services to use a specific user account and grant rights to it.
But I would, if possible, just buy a cheap temporary recording drive for your HTPC.