Hi maxmill,
To create a Luminosity Mask press and hold ⇧ (shift) + ⌘ (cmd) and click on the layer's thumbnail in the Layers panel. Currently this shortcut/command is only available in the desktop version. As suggested above you can create a macro there that performs that step and import it into Affinity Photo for iPad. I've attached the macro file below. Copy it to iCloud or other cloud service you may be using then on Affinity Photo for iPad go to the Macro Studio, click the small menu icon on the header of the Macro Studio panel and select Import Macros..., then select the file you have uploaded to the cloud service.
To use it select the layer you want to apply it to in the Layers Studio, open the Macros Studio, tap the button right below the Macro Studio header to change the category to Luminosity Mask (usually it says Default). You should now see a macro labeled create luminosity mask listed below that category. Tap on it to create a selection based on the luminance of the layer you have selected, then go to the Layers Studio, tap the Mask icon and select New Mask Layer. This will create a luminosity mask from the selection. You can now deselect the selection tapping the canvas with two fingers to display a pop-up menu then selecting Deselect.
