To answer my own question-- Read the fine pod brother! Within minutes of posting, the usual Myers-Murphy Law of Public Posting₁ did it's usual magic. The pod in part reads:

“ The "<perl_codeblock>" directive can be used to parse
curly-brace-delimited block of Perl code, such as: { $a = 1; f() =~ m/pat/; }.
It does this by calling Text::Balanced::extract_codeblock().

If the directive matches text representing a valid Perl code block,
it returns that text. Otherwise it fails with the usual "undef"
value.”

All of which leads to an improved shorter solution:

my $parser = new Parse::RecDescent( q{
startrule:              
    script
script:
    'script' '[' script_options(s /,/) ']' codeblock
codeblock:
    <perl_codeblock>
script_options:
    script_option '=' qstring
script_option:
    'tag-prefix' |
    'language' |
    'implements-prefix' 
is_printable:           
    /[a-zA-Z0-9_&\#;:\$()\'= ,!\@\/.\[\]\-]+/
qstring:                
    '"' is_printable '"'
}
);
[download]

–hsm