Yes, rc.inet1 is executable and it is actually executed during boot.
After hour or so fiddling with init scripts I figured out that problem is somewhere else. There are two network interfaces (besides loopback) - venet0 (NAT) and eth0 (bind to bridge) - and rc.inet1 recognize first network interface (venet0) as eth0, therefore assign IP address to this interface. I'm curious why because of the comment in /etc/rc.d/rc.inet1.conf
# If IFNAME[interface] is unset or empty, it is assumed you're configuring eth<interface>.
while [ $i -lt $MAXNICS ];
However, workaround is simple:
# prepend this to NIC definition
And by the way, if anyone is interested, Slackware 13.37 minimal template
for OpenVZ isn't very good. Missing wget, slackpkg and messed routes in /etc/rc.d/rc.inet1 take some time to get working minimal system. If you want to run Slackware inside OpenVZ, create your own template.