I don't believe the FCV ever fully closes during normal engine operation except at engine shutdown when it goes fully closed to stop air flow into the engine.
During normal operation the FCV only partly closes down to create a low pressure zone in the intake manifold to help facilitate and allow exhaust gases (aka soot) to be pumped into the intake manifold from the EGR valve.
True engine brakes have a valve on the downstream side of the engine in the exhaust pipe that closes off to help with engine braking.
Jake brakes actually close off engine valves to create high resistance to engine turning.
No one uses a valve on the intake side of the engine for braking purposes. As Gordon stated, trying to do so could be dangerous.
The best simple solution is to install a "Block Off" plate on the EGR valve where the exhaust feed tube connects to it.
Long term, it is best to completely remove the entire EGR system with one of the EGR delete kits that are available.
Leaving the EGR cooler in place serves no purpose, it is just more places and hoses that could possibly develop coolant leaks.
See this for EGR "Block Off" plate dimensions:>
viewtopic.php?f=5&t=76576
_________________
Supporting Vendor and Moderator of LOST05 Jeep Liberty CRD Limited
Ironman Springs/Bilstein/Shocks
Yeti StgIV Hot Tune
Week's BatteryTray
No FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pmp
2mic.Sec.Fuel Filter
Flowmaster/NO CAT
V6Airbox/noVH
GM11 Bld.fan/HDClutch
IronrockArms/wwdieselMount98 Dodge Cummins 24 Valve