🚀 go-pugleaf

Hello.  I've been typing mathematics in TeX for a couple years and have not
yet found a way to use various characters from different font families so
that they automatically adjust their size in context.  The following example
will help illustrate my problem:

  I try to use what the AMS refers to as a \Bbb G, in plain TeX (uTeX v3.1a),
without inputting the AMS macros.  I define the font, say \msbm=\msbm10 and
use \magnification=1200.  I need to use the character in both full and
superscript sizes.
   1.  {\msbm\char'107}             gives the expected \Bbb G in 12pt
   2.  $\msbm\char'107$             gives a regular math mode G, that is, a
                                       slanted G in 12pt
   3.  $2^{\msbm\char'107}$         gives 2 with math mode G superscript (the
                                       latter is slanted and proper sized for
                                       a superscript)
   4.  $\hbox{\msbm\char'107}$      gives the expected \Bbb G - 12pt
   5.  2^{$\hbox{\msbm\char'107}}$  gives 2 with \Bbb G in superscript
                                       position, but 12pt

  If I do 1-3 above with regular roman font, that is use \rm\char'107 I can
get "proper" G's and superscript-size.  (4 & 5 of course yield enlarged
superscripts due to the \hbox).  In the past, I've defined different fonts
at the beginning of the papers, i.e., \msbm=\msbm10, \msbn=\msbm8, etc., and
referenced them at appropriate places.  Now I'm facing a book, with unknown
end magnification, and will use many different characters from different font
groups at many different sizes. (And, of course, I spend LOTS of time
in math mode.)  Is there any way to get TeX to treat these other characters
the way it treats roman ones?  Sigh....
   Any help GREATLY appreciated.

            --jessica m.