They preclude ASCII, but not Unicode diagrams.

Unicode block characters have constant width.

Unicode also contains fixed-width equivalents to ASCII letters: ａｂｃ. Just add 0xFEE0 to the code point.

And 0x3000 is an ideographic space, with the same width.

    +---------+
    |         |                        +--------------+
    |   NFS   |--+                     |              |
    |         |  |                 +-->|   CacheFS    |
    +---------+  |   +----------+  |   |  /dev/hda5   |
                 |   |          |  |   +--------------+

turns into

    ＋－－－－－－－－－＋
    ｜         ｜                        ＋－－－－－－－－－－－－－－＋
    ｜   ＮＦＳ   ｜－－＋                     ｜              ｜
    ｜         ｜  ｜                 ＋－－＞｜   ＣａｃｈｅＦＳ    ｜
    ＋－－－－－－－－－＋  ｜   ＋－－－－－－－－－－＋  ｜   ｜  ／ｄｅｖ／ｈｄａ５   ｜
                 ｜   ｜          ｜  ｜   ＋－－－－－－－－－－－－－－＋

EDIT: which would have looked much better had HN not replaced full spaces with the narrow kind.