JIRA 7.3 ships with the Rich Text Editor plugin, and I know it is possible to integrate with that.
Does anyone know whether it is possible to integrate with the Rich Text Editor plugin available on the marketplace when installed in JIRA instances before 7.3 (7.0 through 7.2.x)?
JIRA 7.2 is shipped with Rich Text Editor, but it is disabled by default. You need to follow documentation steps here https://confluence.atlassian.com/adminjiraserver072/rich-text-editing-838542372.html
For versions prior to 7.2 Rich Text Editor can be installed from marketplace https://marketplace.atlassian.com/plugins/com.atlassian.jira.plugins.jira-editor-plugin/versions
Please be aware that Rich Text Editor for versions before 7.3 does not contain all bugfixes and does not include API for third party plugins integration.
I will strongly recommend updating to JIRA 7.3 to get the best Rich Text Editor experience.