Hi,
The HTML helper is a good way to do it, as the Nop Mega Menu uses its own MegaMenuCategoryModel which is optimized in terms of performance. In the beginning we used the nopCommerce's category model, but there were a lot of properties and models that were built as part of the category, which were not used by the Nop Mega Menu. That is why we use our own model that contains only the properties we need for the Nop Mega Menu. This is the reason why your new field is not populated correctly in the Nop Mega Menu, because we use our own model and controller which does not set anything extra.
Hope that makes sense!