I suspect that the reason Synology hasn't added L2TP is the performance hit it would have on the NASes. IPSEC uses 3DES and this could have major impact on any NAS that can't encrypt/decrypt 3DES in hardware.
Possibly it may be fine on any of the + or xs models, but may have a severe impact on any of the standard and j models.
Imagine someone who has a NAS that turns on nearly every feature of the NAS, sure it says it can do all these wonderful things on the box, but trying to do alot of these things simultaneously is bit much to ask of the slower CPUs.
I am only speculating here.
"The NASman - Storage for your digital future"
Synology Model: DS1511+, CS407e
Firmware: 1358, 1285
HDDs: 4 x Samsung HD204UI 2TB
HDDs: 4 x Samsung HD501LJ 500GB
DMAs: 1 x XBMC,1xWDTV Live