I believe it is exactly what you found.
on the following posts a Mikrotik Support - MrZ stated
RouterOS does not support tunnel password which is used for LAC/LNS.
http://forum.mikrotik.com/t/mikrotik-l2tp-client-to-juniper-device/68795/12
in the v5.07 release notes it did hint that it is already supported.
What’s new in 5.7 (2011-Sep-14 10:54):
*) ipsec - new exchange mode (main-l2tp) for l2tp tunnel users to allow
FQDN as a peer ID with preshared key authorization in main mode;
But haven’t as yet been able to see how to set that up, or if that even hints at the same thing,
[flash=]Would make my life so much easier if i could implement this.[/flash]