I tried running this command
rpm -U --prefix /data1/builds/1 \
I have a topdir setup in /data1/builds/1 and the
files did not go into /data1/builds/1/SOURCES and
I use RPM version 2.5.6 so this may be fixed in your sources
The --prefix option supplies a new leading path for a tag in a binary
rpm containing that contains the old leading path and absolute
path names. Since src.rpm's contain relative not absolute paths
and do not contain a prefix tag at all it's unclear whether the
effort involved in implementing --prefix is worth the effort.
rpm --define '_topdir /data1/builds/1' -U ...
which should have the desired effect.