Could 'indoors' and 'outdoors' not represent both a man and a woman respectively?
Poem symbolises a relationship between a man and a woman, 'tiny oil lamp', 'red blood cools' = phallic imagery, whilst 'empty deep' and 'bowl of heaven' = yonic imagery. 'Winking signal' = provocative tone
'chill', 'void', 'siren' = semantic field of warning, supported by 'indoors the locked heart and the lost key'.
There is some danger to using a system of Freudian interpretation in reading this poem. As Freud himself said, "Sometimes a cigar is just a cigar."
In "House on a Cliff" by Louis MacNeice there is some traditionally feminine imagery in the outdoors as you have noted – the bowl of heaven and feminine moon and sea – but these are both traditional usages. The moon, in western culture, is always female (and associated with female goddesses) and the sea and tides are normally personalized as feminine. This doesn't really mean that the author is thinking of the outside as a woman, but perhaps that the gender bifurcation has more to do with the contrast of the lonely aging man and the eternal possibilities of the outdoors. Female companionship is one of the many things the man lacks (the "locked heart" is suggestive), adding another layer to poignancy to the imagery.