I have some ideas to point you in a direction at least... I've only started working with the code this weekend, but it's pretty straight forward.
You'll want to do a check for if browsing a specific category, then write an if / else to change the button. Depending on whether you have a button image, or if you are still using the TXT variables.
If using the TXT variables with a background images, then you'll want to go to the languages first and define a new variable to use that says "Call to Order".
Then, you'll write you're if to choose between something like
<input type="submit" value="<?php echo TXT_WPSC_ADDTOCART; ?>" name="Buy" class="wpsc_buy_button" id="product_<?php echo wpsc_the_product_id(); ?>_submit_button"/>
-OR-
<input type="submit" value="<?php echo TXT_WPSC_CALLTOORDER; ?>" name="Buy" class="wpsc_buy_button" id="product_<?php echo wpsc_the_product_id(); ?>_submit_button"/>
Your If / Else will be something like:
<?php if(wpsc_is_in_category('10')) : ?>
And I think you'll have to do this in both the products_page.php and the single_product.php.
Hopefully that gets you started.