Thanks BrowserUK!!!!! That's it!!

Just before I came back and checked here, I was finding that it was the instance of the interpreter so I was just about to head down that path. You made it extraordinarily easy for me. I just had to add two lines (the PerlInterpreter line and the PERL_SET_CONTEXT line. Here's the working pthread test script. After I clean up my original code that started this thread, I'll update the thread.

use warnings;
use strict;

use Inline (C => 'DATA', libs => '-lpthread');

create_thread('blah');

sub blah {
    print "perl callback\n";
}

__DATA__
__C__

#include <stdio.h>
#include <pthread.h>

PerlInterpreter *saved;

void wrapper(void *sub_name_ptr){
    char *sub_name = (char *)sub_name_ptr;

    PERL_SET_CONTEXT(saved);

    printf("threaded ok, sub: %s\n", sub_name);

    dSP;
    PUSHMARK(SP);
    PUTBACK;

    call_pv(sub_name, G_DISCARD|G_NOARGS);

    FREETMPS;
    LEAVE;

    return NULL;
}

int create_thread(char *subname){   
    pthread_t sub_thread;
    
    if(pthread_create(&sub_thread, NULL, wrapper, subname)) {
        fprintf(stderr, "Error creating thread\n");
        return 1;
    }
    
    if(pthread_join(sub_thread, NULL)) {
        fprintf(stderr, "Error joining thread\n");
        return 2;
    }
    return 0;
}
[download]

Hi, um, :) https://metacpan.org/pod/distribution/perl/pod/perlguts.pod#How-multiple-interpreters-and-concurrency-are-supported

So to the top add #define PERL_NO_GET_CONTEXT     /* we want efficiency */ and then

void callback(){
dTHX;       /* fetch context */
...
[download]

and thats it?

So to the top add #define PERL_NO_GET_CONTEXT

PERL_NO_GET_CONTEXT doesn't work with Inline::C.
You can do pre_head => '#define PERL_NO_GET_CONTEXT 1', and that will get the directive inserted at the very beginning of the generated XS file (where it needs to be).
But Inline::C is not presently capable of generating XS code that meets the requirements of having PERL_NO_GET_CONTEXT defined.
I filed a bug report about this some time back.

InlineX::C2XS contains a rather flakey, fragile, sub-standard hack that does accommodate PERL_NO_GET_CONTEXT.
It's functional enough for my purposes, but it does impose some limitations on the way one formats one's C code.

Cheers,
Rob