Is it possible to modify the boundaries within a SIP-Manipulation?

Does anyone know if the boundaries that separate the application bodies within the multipart/mixed content can be modified with sip-manipulations?

We tried using the below manipulation but in testing, the boundaries themselves were never modified:

sip-manipulation

name Addr_n

description

split-headers

join-headers

mime-rule

name AddReturnToLastBroundary

content-type application/pidf+xml

msg-type any

methods INVITE

format ascii-string

action find-replace-all

comparison-type pattern-rule

match-value \r\n--([a-z0-9]{32})--

new-value "\r\n--"+$1+"--\r\n"

testing this on the acme packet 3950 SCZ9.2.0 Patch 4 (Build 156)