1

Is it possible to create a tag mask dropdown similar to the layer mask dropdown in a custom inspector, to be able to select multiple Tags?

EDIT:
the tags are not known in advance.

2

Sure. Just use the EnumMaskField, with a custom enum of your choice.

Make sure the enum uses correct values for the bitmask:

Example:

private enum MyEnum
{
    MyValue1 = 2,
    MyValue2 = 4,
    MyValue3 = 8,
    MyValue4 = 16,
    MyValue5 = 32,
}