1.7.10 might not have the code required to do insertions or something might have been changed on how it works :/
And yes if there is a server command that uses /// my mod will supersede it because the command is client side and run first before attempting to even send the command to the server.
Oh so you're using the vanilla "tellraw" command?
1.7.10 has Tellraw I'm pretty sure, maybe it doesn't have the append part yet.
It'd be cool if you worked around that but not essential (since there's 1.10.2 and stuff)