First, it'll be very marginal, at least the long is impossible to appear in classic onyomi (, , ) series and "regular" native words. is like "mom" as opposed to "mother", and you can see these kind of words rarely get an established kanji.
Below is the all results I get through prefix search on a J-J dictionary ( _italic_ is my own addition).
* ****
* **** (cf. )
* **/**
* ****
* ****
* ****
* ****
* ****
* ****
* ****
* ****
* **__**
* **** __ ( etc.)
* **/**
* ****
If you include proper names like and , the number would be nearly infinitive. There are also Western loanwords like and , but not quite established.