Per some of the mustang forums I frequent, this is one method of getting around the TPMS system.
1. You will need the 4 TPMS sensors removed from your wheels. If you are having non-runflats installed or a new set of rims with non-runflats, this should be easy.
2. Go to the hardware store and find a piece of pvc pipe large enough to fit all 4 TPMS sensors. Purchase end caps and some PVC glue or screw on end caps and the proper PVC pipe with at least one screw-on end.
3. Purchase a valve stem from a tire shop or bike shop.
4. Glue one endcap onto one end of the pipe.
5. Drill a hole in the other endcap just large enough so that the valve stem fits snugly.
6. Glue the valve stem in place so it is airtight.
7. Put your TPMS sensors in the PVC pipe.
8. Glue or screw the other endcap onto the PVC pipe.
9. After the glue has set, pressurize the PVC pipe using an air compressor or tire pump to the recommended tire pressure per the owners manual.
10. Secure the PVC tube somewhere safe in the trunk.
11. Reset the onboard TPMS.
12. Monitor your tire pressures with a standard tire pressure gage from time to time.
Here are some sites for reference.
http://www.scionlife.com/forums/viewtopic.php?p=3153329
http://www.mustangforums.com/m_2060535/tm.htm
Hope this helps, Nate.