There is no option for that at the moment on the page for configuring the wireless interface, but you could change the script to add the parameter. In /etc/inc/interfaces.inc go to the bottom of the interface_wireless_configure function. There you should see this:
/* configure wireless */
$wlcmd_args = implode(" ", $wlcmd);
mwexec("/sbin/ifconfig {$if} $wlcmd_args", false);
Before the $wlcmd_args = implode(" ", $wlcmd); line you can add ifconfig parameters to $wlcmd. If you only have one wireless interface assigned you could change it to this (replace your_ap_mac_here with the AP MAC, with colons as the separator):
/* configure wireless */
$wlcmd[] = "bssid your_ap_mac_here";
$wlcmd_args = implode(" ", $wlcmd);
mwexec("/sbin/ifconfig {$if} $wlcmd_args", false);
If you have multiple wireless interfaces, you could do something like this (assuming ath0 is the assigned interface for the example):
/* configure wireless */
if ($if == "ath0_wlan0")
$wlcmd[] = "bssid your_ap_mac_here";
$wlcmd_args = implode(" ", $wlcmd);
mwexec("/sbin/ifconfig {$if} $wlcmd_args", false);
