I did something similar - commented out the @import command from the WP child theme css so it's essentially blank. Everything seems to work!

Code:
RewriteEngine On

# Don't rewrite any URIs ending with a file extension (ending with .[xxxxx])
RewriteCond %{REQUEST_URI} !\.[a-z]{2,5}$ [NC]
# Don't rewrite admin directory
RewriteCond %{REQUEST_URI} !^/myadmindir [NC]
# Don't rewrite editors directory
RewriteCond %{REQUEST_URI} !^/editors [NC]
# Don't rewrite cPanel directories
RewriteCond %{REQUEST_URI} !/cpanel [NC]
RewriteCond %{REQUEST_URI} !/frontend [NC]
# Don't rewrite blog directory
RewriteCond %{REQUEST_URI} !^/blog [NC]
RewriteCond %{REQUEST_URI} !^/blog-backend [NC]

# Handle all other URIs using Zen Cart (index.php)
RewriteRule .* index.php?%{QUERY_STRING} [L]

# Do rewrite blog directory but not wp directory
RewriteCond %{REQUEST_URI} !^/blog-backend [NC]
RewriteRule ^blog(.*)$ index.php?main_page=blog&$1 [E=VAR1:$1,QSA,L]
I feel like I could have done this with one less directive by modifying and moving the last two to before the "all other URIs" directive, but hey, it works.