I have a RB411 which has one ethernet and one wireless interface. My internet access is handled by a Billion 7404VGO ADSL router. My PC (192.168.0.1) and the RB411 (192.168.0.100) are both plugged into the ADSL router (192.168.0.254) and they all have static IPs assigned. All the devices can ping each other. The default gateway on the RB is set to my ADSL router's IP.

I'd like to test the hotspot functionality on the RB. If I understand everything correctly, I'd normally use eth0 (192.168.0.100) as the "wan" link, and wlan1 as the "lan" (for hotspot users) link, but since I can't use wlan1, is there any way to add a second, "virtual" ethernet interface for which I can then assign an IP on another subnet (say, 10.0.0.1/24) with the idea to then change my PC's IP to 10.0.0.2/24 in order to get to the captive portal page when I try to browse to a website?

The ADSL router seems to support VLANs (not sure if that'll even help) and I have the following options available in its configuration:

Configuration -> LAN -> Bridge interface:
Configuration -> Advanced -> VLAN Bridge:

So you have the rb411 connected to one port of the 7404, and your pc connected to another port of the 7404 (and these ports are presumably bridged together)?

If the 7404 supports it, you need to make a vlan on the rb411, with your 10.x.x.x network, and make that a tagged vlan on the 7404, then the port with your pc needs to be in that same vlan, but untagged.

example:
rb411 ether1 <-> 7404 ether1 192.168. network
rb411 ether1.vlan10 <-> 7404 ether1.vlan10 10.x network tagged
pc ether1 <-> 7404 ether2.vlan10 10.x network untagged

Alternatively, you may have to tag the 192.168 network on a different vlan, depending on how the 7404 works.
rb411 ether1.vlan9 <-> 7404 ether1.vlan9 192.168. network

Be careful to not lose management access to your devices setting this up. You could mac-telnet via winbox to the rb411 if necessary; don't know about the 7404.

The VLAN settings on the Billion doesn't make much sense to me at this stage, but that's probably because I don't yet understand the difference between "tagged" and "untagged". I believe I have a bit of reading up to do now.

Thanks for letting me know that it should be possible, though, and for the example!