Unlike most other routers on the market, there's no specific "PPTP Passtrough" option in the 7402GXL, but the method to allow this on the Billion router is quite simple:
Under the Virtual Server > Port Forwarding section, define a redirection for PPTP port and another for GRE protocol pointing to the intranet IP of the VPN server.
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And it the firewall is enabled, define an rule to allow the PPTP port 1723 to go through (In and Out)
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In theory, this should have done the trick, but not for me: despite the troublesome firewall being off, the VPN client was still hitting an invisible wall, and there was no log entry whatsoever in the router or at the VPN server end to give any clue.
Resolution
I contacted Billion technical support to check for any newer firmware for my model, which was in 6.22b (which, despite being 6 months old is only confidentially mentioned on the UK forum).
They provided me with the version 6.24b(UK build). It appears to be one of the latest available (January 2012) according to the Australian site (no changelog available).
Once installed, the VPN connection worked immediately with the same set of rules and forwarding.
So, it is possibly a bug that the latest firmware eventually resolved. Of course, it is also possible that resetting the router to factory settings -which is a requirement when upgrading a firmware for Billion's devices- cleaned up something corrupted in the configuration.