If you want to code your own custom tag extension and package it as an TYPO3 plugin, there could be some issues getting things to work in RTE enabled textarea fields. The Extension Kickstarter only supports to add the necessary TypoScript setup for the tt_content.bodytext field but no configuration for proper redisplaying of the tag in RTEs is made.

With this setup (PageTS-Config and TypoScript Setup) you will be able to redisplay the tag in the RTE and also render it nicely with your plugin code.

Just create an extension with the Kickstarter, insert YOURTAGtag as extension key and replace YOURTAG with the tag name you like (also in the extension key).

Note: the PageTS-Config is non-destructive concerning already existing configurations, just insert it after your (propably customized) RTE configuration in the TSConfig field on your rootpage. Only settings I've found out to be necessary were added. Leave me a comment if something is redundant or unnecessary. Thanks!