How can I get the new secure trading gump to display the silver and gold amounts?
I assume it'll require some special packet to be sent to the client.

I'm interested in this too

Is that a custom gump or client-builtin feature?

That's the new secure trading that I believe was released with ToL.

I'm also interested in getting this new style S.T. gump to display the info it appears to have spots for.

As ThisIsMe said it's the new secure trade gump in the Time of Legends expansion.

I suspect there's additional data that needs to be included in SendSecureTrade (or whatever its called) gump packet.

When it comes to packets and packet building I know even less about that than I do about advanced computer programming which means I know nothing about building packets.

I don't play the "pay for" official servers so someone who does will need to do some "packet sniffing" to get the info being sent to the client. I will tell you I didn't have that gump pic in my gumps UOP file because I was using a custom gump.mul file rather than the one that came with ToL and when I initiated a secure trade no gump appeared. So that tells me that they are using the new gump now.