Nope. The most you can do is adjust the alignment of the header or description text, but can't make it bold, italic, etc. (header text is already bold if I recall correctly)
Technically cocoaDialog can't do it either. The only "bold" you can do with it is when using the msgbox window style and using a --text line, but its limited to a single line, so if your text is too long for the window width it just gets chopped off.

I would love love love to see a utility that could allow for full customization, or at least more customization of the dialog styling. There really isn't anything out there that can do it. Pashua allows more fine grained control over the positioning of the text, icons and other elements, but its not all that easy to use, and doesn't really lend itself well to a simple scripted policy.

I figured and was afraid of that, however just wanted to make sure...

thanks for the quick response @mm2270

You can use an AppleScript dialog box to display a bold text by maintaining the window type "alert":

osascript << EOF
tell application "Self Service"
    activate
    display alert "Place your bold text here" message "place your message here" buttons {"OK"} default button 1
end tell
EOF

Is there a way to do this in the Message Box now that JSS is up to 10.3?
Future request if not.

You probably shouldn't, but you sort of can via (ab)use of Unicode:

/Library/Application Support/JAMF/bin/jamfHelper.app/Contents/MacOS/jamfHelper  -title "𝐛𝐨𝐥𝐝, 𝒊𝒕𝒂𝒍𝒊𝒄, etc" -windowType utility -description "make the text either 𝗯𝗼𝗹𝗱, 𝘪𝘵𝘢𝘭𝘪𝘤, or u̲n̲d̲e̲r̲l̲i̲n̲e̲d̲" -heading "𝖄𝖊 𝕺𝖑𝖉𝖊 𝕸𝖊𝖘𝖘𝖆𝖌𝖊"

See Unicode Toys