Their are 4 possible routes into Central Asia via Europe:
Across the black sea, from the Northern steppes, from the South-East(Afganistan) and from the South west(Turkmenistan).
The former 2 are easily closed off and domnated by a powerful united Russia, the later 2 by a powerful and united Iran. Nor the Afghan route effective as an independant route from the modern dar territories of Iran.
Thus, I see only a couple of ways to do it.
Have the the Latin Empire survive, thus a France-Venetian Alliance also survives. They eventually create a Volga canal, and set up French and Venetian colonies.
Russia remains fragmented, and Sweden vassalizes much of it, using it as a route to Central Asia(you can have a united or fragmented Russia do this, but that wouldn't be Western, would it?)
The crusader states conquer Northern Persia(stretching it).
Or just change the defintion of Western European power
.