I tested this and it works (the way I expect, at least). Here is an example tmsh command for creating the Data Group:
tmsh create ltm data-group internal dg-route-blocks-to-remove \
type ip records add { 10.10.210.0/24 { } 192.168.5.0/25 { } }
Naturally, you would substitute your own set of netblocks.
Do keep in mind that this mechanism will always move the Route header to the end of the message header set (in 11.6, using the SIP::header replace method avoids this).
As an example, using the Data Group definition above, I sent the following SIP Request through a Virtual Server with the iRule above attached:
REGISTER sips:ss2.biloxi.example.com SIP/2.0
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92
From: Bob ;tag=ja743ks76zlflH
To: Bob
Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
CSeq: 2 REGISTER
Route: ,,,
Content-Length: 0
and this is the message that arrives at the Pool Member:
REGISTER sips:ss2.biloxi.example.com SIP/2.0
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92
From: Bob ;tag=ja743ks76zlflH
To: Bob
Call-ID: 1j9FpLxk3uxtm8tn@biloxi.example.com
CSeq: 2 REGISTER
Content-Length: 0
Route: ,