Answer
Yes — when the filter condition needs to reference a custom field, make sure you're building the Lobby element's data source against the entity's custom-field view (ending in _CFV), not the standard base view, and reference the actual custom field column there (for example, CF$_C_STOCK_LEVEL) tied to your Lobby parameter (for example, '$Quantity$'). Referencing the custom field through the correct _CFV view is what makes a custom-field-based Lobby filter actually work.