Hi. For my experience, if you need to use DSP, the only way is the E1 loop as you described.
I used this solution to convert RTP passthrough FAX to T.38.
Anyhow, if you don't need to pass the call to internal DSP, you can use the IP2IP feature:
conf t
voice service voip
allow-connections sip to sip
In this case the AS5400 acts as a B2BUA so you will have a scenario like this:
SIP call ----> AS5400 ----> SIP call
You can also use the media flow-through to have 2 RTP flows or media flow-around to bypass the AS:
SIP call ----> AS5400 ----> SIP call
RTP flow ---> AS5400 ----> RTP flow
SIP call ----> AS5400 ----> SIP call
RTP flow ------------------> RTP flow
Regards.