ALTQ on incoming interface

Sometimes there is really need for ALTQ on incoming traffic:

--------
| SRV1 |
| ftp |
--------
|
| ------------ ------------
+------| |$ext_if | |
$int_if| BSD1 box |---IP sec only---| BSD2 box |----Clients
+------| | | |
| ------------ ------------
|
--------
| SRV2 |
| smtp |
--------

Example:
If ftp traffic to the client is large, then smtp traffic
will be blocked. There is obviously need to queue the
traffic in some a way. (All SRV, BSD and clients
are in company, they are not "strange" internet machines).

ALTQ on enc0 is not possible (it is well known).
There is problem on queue on $ext_if (it is only IP sec traffic).

Is there any Idea/Solution how to solve this?


My Idea is as follows: Use route-to loopback interface
on incoming traffic to internal interface.
Then apply queue on outgoing traffic from lo0.

Part of /etc/pf.conf is something like:

pass in on $int_if route-to lo0
pass out quick on lo0 from $SRV1 to any queue ftpque
pass out quick on lo0 from $DRV2 to any queue smtpque
pass out on lo0 all

Is it reasonable? Any comment will be appreciated.

Igor