JFTR, I ended up adding this in debian/rules:
PY_VERSIONS = $(shell pyversions --requested debian/control)
install-stamp-%:
cd $(DEB_BUILDDIR) && rm CMakeCache.txt && \
cmake $(DEB_CMAKE_NORMAL_ARGS) $(DEB_CMAKE_EXTRA_FLAGS) \
-DPYTHON_EXECUTABLE:FILEPATH=/usr/bin/$*
make -C $(DEB_BUILDDIR) install DESTDIR=$(CURDIR)/debian/tmp
touch $@
install/python-ufc:: $(addprefix install-stamp-, $(PY_VERSIONS))
Johannes