If I’m missing another way of rendering checkboxes in PmWiki→Wiki Publisher, please let me know.
This is not a good idea. In principle, Wikipublisher should translate forms markup into the corresponding PDF form constructs. At the moment PDF forms are not supported and there are no plans to add this, as the primary target is print and to date nobody has made a case for adding forms capability. However, we should keep the option open for the future.
A better solution may be to use a customised list, replacing the default bullet symbol with a box. See for example http://en.wikibooks.org/wiki/LaTeX/Formatting#Customizing_Itemised_Lists. An author would apply a custom wiki style to the list and Wikipublisher would translate this into a suitable list. Unfortunately, HTML CSS does not provide an “open square” option, although there is an entity for this: ☐ — not sure if it is possible to use an entity in a list style.
Meanwhile, try using ☐. At the moment, there is a bug in templates/tbents.txt at line 1160 which prevents Wikipublisher from correctly translating this entity into the LATEX equivalent. It should read: $\Box${} rather than \Box{}.