  For any semifinite von Neumann algebra ${\mathcal M}$ and any $1\leq
p<\infty$, we introduce a natutal $S^1$-valued noncommutative $L^p$-space
$L^p({\mathcal M};S^1)$. We say that a bounded map $T\colon L^p({\mathcal
M})\to L^p({\mathcal N})$ is $S^1$-bounded (resp. $S^1$-contractive) if
$T\otimes I_{S^1}$ extends to a bounded (resp. contractive) map
$T\overline{\otimes} I_{S^1}$ from $ L^p({\mathcal M};S^1)$ into $L^p({\mathcal
N};S^1)$. We show that any completely positive map is $S^1$-bounded, with
$\Vert T\overline{\otimes} I_{S^1}\Vert =\Vert T\Vert$. We use the above as a
tool to investigate the separating maps $T\colon L^p({\mathcal M})\to
L^p({\mathcal N})$ which admit a direct Yeadon type factorization, that is,
maps for which there exist a $w^*$-continuous $*$-homomorphism
$J\colon{\mathcal M}\to{\mathcal N}$, a partial isometry $w\in{\mathcal N}$ and
a positive operator $B$ affiliated with ${\mathcal N}$ such that
$w^*w=J(1)=s(B)$, $B$ commutes with the range of $J$, and $T(x)=wBJ(x)$ for any
$x\in {\mathcal M}\cap L^p({\mathcal M})$. Given a separating isometry $T\colon
L^p({\mathcal M})\to L^p({\mathcal N})$, we show that $T$ is $S^1$-contractive
if and only if it admits a direct Yeadon type factorization. We further show
that if $p\not=2$, the above holds true if and only if $T$ is completely
contractive.

Operator Algebras