I don't think you need to support the BS-X BIOS stuff. You just have to map the ROM in the banks $80-$bf differently.
Relevant part of xml.cpp:
Code
    xml << "  <rom>\n";
    xml << "    <map mode='linear' address='00-1f:8000-ffff' offset='000000'/>\n";
    xml << "    <map mode='linear' address='20-3f:8000-ffff' offset='100000'/>\n";
    xml << "    <map mode='linear' address='80-9f:8000-ffff' offset='200000'/>\n";
    xml << "    <map mode='linear' address='a0-bf:8000-ffff' offset='100000'/>\n";
    xml << "  </rom>\n";
    xml << "  <ram size='" << strhex(ram_size) << "'>\n";
    xml << "    <map mode='linear' address='70-7f:0000-7fff'/>\n";
    xml << "    <map mode='linear' address='f0-ff:0000-7fff'/>\n";
    xml << "  </ram>\n";
    xml << "  <bsx>\n";
    xml << "    <slot>\n";
    xml << "      <map mode='linear' address='c0-ef:0000-ffff'/>\n";
    xml << "    </slot>\n";
    xml << "  </bsx>\n";
Forum post on the zboard:
http://board.zsnes.com/phpBB2/viewtopic.php?p=169088#169088