Further down the page is a mention of Flex, which was designed to address the concerns you mention:

    "Windowgrams are a neat way of arranging workspaces. For simpler
    layouts, a windowgram can be typed up quickly. But if you need
    detail, you may find yourself doing a lot of ASCII art.

    In tmuxomatic 2.x, flex was added to automate the construction and
    modification of the windowgram itself.

    Flex is an object-oriented windowgram editor. It is visually
    expressive, naturally worded, logically ordered, minimal, and
    powerful. Its short command set may be combined to make any
    conceivable windowgram -- likely more quickly and more easily than
    crafting by hand. Flex is intended for power users who desire
    detailed windowgrams without the tedium of manual entry."

There are some examples further on of how quick and easy it is to use flex for edits.