Path variables are typically : delimited. White space is allowed in paths, so is not a good choice for separating paths. Currently utils.bbclass performs the following:

def base_set_filespath(path, d):
	filespath = []
	extrapaths = (bb.data.getVar("FILESEXTRAPATHS", d, True) or "").split()
	path = extrapaths + path
	# The ":" ensures we have an 'empty' override
	overrides = (bb.data.getVar("OVERRIDES", d, 1) or "") + ":"
	for p in path:
		for o in overrides.split(":"):
			filespath.append(os.path.join(p, o))
	return ":".join(filespath)

This splits FILESEXTRAPATHS on whitespace and then splits overrides on : and reassembles them all together as : delimited.

There is only one user of FILESEXTRAPATHS in oe-core (which uses : anyway), none in OE:

dvhart@doubt:~/source/oe.git
$ find . -name "*.bb*" | xargs grep FILESEXTRAPATHS
<EMPTY>

dvhart@doubt:~/source/oe-core.git
$ find . -name "*.bb*" | xargs grep FILESEXTRAPATHS
./meta/recipes-qt/qt4/qt4-tools-native_4.7.3.bb:FILESEXTRAPATHS =. "${FILE_DIRNAME}/qt-${PV}:"
./meta/classes/utils.bbclass:	extrapaths = (bb.data.getVar("FILESEXTRAPATHS", d, True) or "").split()

It should be acceptable to simply change the implementation.