> People would need to use tabs (gasp) instead of spaces for that :-P

That would be madness.

You could just do a naive 4spaces to 1tab conversion.

This breaks for spacing between non-whitespace character. E.g. the list of flags, which may have varying lengths, but all descriptions start on the same alignment.

    This is a man page with a list of flags below

    --flag1        Description of flag1
    --secondflag   Description of secondflag

If we naively replaced the above with 4spaces=1tab we'd get the whitespace between flag1 and its description replaced with two tabs, and we'd get the whitespace between secondflag and its description left as a string of three space characters.

I have no idea what is actually used in man pages for alignment and spacing, but the example above is only to illustrate that 4space=1tab breaks even on trivial examples as a conversion rule.

That does not matter for single dash and letter options. For double dash long options I like to split them on different lines so that you still have a lot of horizontal space for the descriptions themselves even with very ling option names, so a brute force way:

  This is a man page with a list of
  .I flags
  below
  .P
  .BR --flag :
  .RS
  Description of flag1
  .RE
  .P
  .BR --secondflag :
  .RS
  Description of secondflag
  .RE
  .P

Also for real tables:

  $ man tbl