Multiple VLANs on a Synology NAS

Synology, like other SOHO/SMB NAS vendors, touts VLAN functionality with their current DSM 4.1 software. However, the web interface just lets you specify one VLAN tag to use over each eth interface (or bond interface).

Manual approach

In the busybox environment that you can ssh into as root (after enabling ssh through the webinterface), there's all the tools you need to use multiple VLANs over one link (eth or bond), however:
First you insert the 802.1q module into the Linux kernel:

 /sbin/lsmod | /bin/grep -q 8021q || /sbin/insmod /lib/modules/8021q.ko

Then you add each VLAN you need to every interface (bond0 in this example)

 /sbin/vconfig add bond0 4

And finally you can configure IP addresses on every interface.vlan combination (bond0.4 in this example)

 /sbin/ifconfig bond0.4 192.168.4.1 broadcast 192.168.4.255 netmask 255.255.255.0

The same type of script would work on a QNAP NAS too, by the way. They offer 8021q.ko and vconfig in their commandline environment as well.
Packets from the bond0 interface leave the device untagged, packets from the bond0.4 interface leave with a tag specifying VLAN 4.
Be aware that these settings only last until the next reboot.

Synology approach (future?)

Synology has its own set of utilities that are used by the webinterface to manage devices. The network interface settings are managed by /usr/syno/sbin/synonet. This utility sets up bonded interfaces, IP addresses, and VLAN entries. However, the utility has the same limitations as the web interface (for unknown reasons): creating a VLAN unconfigures the untagged interface you're working on, and you can't add a second VLAN on the same interface.
It would be nice if synonet could get multi-VLAN support, as all the necessary options seem to be there already. Feature request, Synology?