Maybe that's a (intended) limitation of NFS in Solaris 10*. Maybe you have to set some option in order to export a filesystem across filesystem borders. For performance reasons you should mount /sp from machine A directly on C (way over B doubles network load and esp. locking latency).

Despite some special situations mounting /tmp via NFS is not that good as
(i) it's not possible to have sockets on NFS (some apps depend on such things in /tmp)
(ii) some apps do write quite heavily to /tmp (and get slow that way)
(iii) if some system deamon uses a temp file and your network breaks or gets slow your system gets into trouble

* to find out whether solaris or hp-ux doesn't do it: can you mount B:/tm on A (in some other directory)?