Yes, other plugins will install and work. I have multiple sites on Pantheon and many of them have multiple plugins. This is the first issue of this type I have had with a plugin on Pantheon.
I tried adding the config file setting and received a new error.
Notice: Use of undefined constant ‘GEODIR_USE_PHP_SESSIONS’ – assumed ‘‘GEODIR_USE_PHP_SESSIONS’’ in /srv/bindings/4f39a1f7816543bca1e65735836035bc/code/wp-config.php on line 151 Warning: session_start(): user session functions not defined in /srv/bindings/4f39a1f7816543bca1e65735836035bc/code/wp-content/plugins/geodirectory/geodirectory-functions/geodir-class-session.php on line 280 Fatal error: session_start(): Failed to initialize storage module: user (path: ) in /srv/bindings/4f39a1f7816543bca1e65735836035bc/code/wp-content/plugins/geodirectory/geodirectory-functions/geodir-class-session.php on line 280
If I can get this working and demo it to my client then they will buy the subscription package.