I had a very similar situation with a package, and with AdminStudio quirks. The machine name needed to read dynamically during the install. Yes, I finally figured out that you use "[%computername]" for that, but I had tried replacing the Property Clientname, in this case, with [%computername], but when Clientname was used to set a registry entry, of course it put in literally "[%computername]". So I either had to put "[%computername]" in for the registry entries (not a very good way of doing it) or use "[[Clientname]]" so that the value of Clientname was inserted, and then THAT was interpreted to be the actual machine name.
The AdminStudio quirk was this: I did all that and it still wasn't working. I was modifying the MST in Developer because of the additional capabilities it offers over Tuner. Everything I tried, the registry was always stamped with literally "[%computername]". Finally, I opened the same MST in Tuner, and looking at the same fields in there, it wasn't "[%comptuername]", but something more like "[[/]%computername[/]]". It did NOT show up that way in Developer and was driving me nuts. Just thought maybe someone else might find that useful or may have seen the same thing before.