Pl/tcl auto-load problem

How do I use the pltcu unknown module to auto-load a return value from a
select query?  I'm trying to boost performance in our databases by
preloading
certain frequently-used variables. Ultimately I'd like to auto-load
prepared queries this way too.

I've figured out how to use pltcu to auto-load a global variable and access
that variable by appending the following lines to unknown.pltcl:
    global var1
    set var1 abc
Then, in a pltcu procdure, I can access it:
    create function get_var1() returns varchar as '
       global var1
       return $var1
    ' language 'pltcu';
(returns abc)

What I'd like to do is something like the following in unknown.pltcl:
    global ret
    spi_exec "select col1 from tablename where col2 = ''xyz''"
    set ret $col1
and return the global $ret in pltcu functions.

When I try use spi_exec in unknown.pltcu, I get an error when loading the
module:
    New/updated unknown: too many nested calls to Tcl_EvalObj (infinite
loop?)
    Abort

I've tried a few other variations, all without success. Any suggestions?
(I'm
using pltcu because I need a trigger function that can write to system
files, and I don't think that can be done with plperl or plpython.
Performance
boosting through hardware upgrades is not a viable option because the
databases are on tablets running Linux with 245 MB ram and damnably limited
battery life.)

Craig Addleman
DBA
www.sharechive.com