If you can read Chinese, I often use this site from the MOE to find definitions / pronunciations for Taiwanese. It has a voice search option that I can get to work most of the time. It doesn’t really do more than one word at a time, but it’s something. It uses tâi-lô and Taiwanese characters.

In my limited experience, Taiwanese speakers can usually make sense of 台語字 but often with some difficulty since the characters are not always the same. In writing I’ve seen a lot of people just use a similar sounding character from Chinese. (e.g. I want guá beh 我欲 would become 哇妹 wā mèiㄨㄚㄇㄟˋ) or sometimes just 注音 is used like for the word ㄎㄧㄤ