You can find any plaintext table generator on github and replace the padding character they use. I've done it with this one:
https://github.com/ozh/ascii-tables (Go to script.js, and on line 418 replace the space with whatever unicode character you want).
Unfortunately that still won't work, because jsp displays text in regular posts using a variable-width font (as opposed to a monospaced/fixed-width font), which means that you can't easily pad columns by making sure every column has the same number of characters (which is what plaintext table generators do).
The simple solution is to use the code tag which renders text in a monospaced font, but of course bbcode won't work inside (although you wouldn't need to bother with unicode spaces then).
You might be able to measure the string width in actual pixels using the canvas
https://stackoverflow.com/a/21015393/4500419 and use that to pad columns to the appropriate size, but it'd be super hacky and probably wouldn't work that great, would have lots of bugs depending on the browser used, if the user has a custom stylesheet applied on top, etc.