If there is a lag time between exposure and PO image formation, that would be very interesting.

However, mask formation inherently leads to less contrast, not more... right? If you think about these two mechanisms, reciprocity failure and self-masking, they're actually working against each other.

Furthermore, if indeed there is some lag time between exposure and PO image formation, by slowing down the printing time the effect would again be less contrast, not more. This lag could give us more contrast only if we get in a lot of exposure before the PO mask begins forming, which would only occur with a faster printing time (assuming we're beating the lag to some degree). So slower printing time should equal a more direct relationship of exposure to PO image formation (less effect of lag), and thus stronger masking, i.e. less contrast. However, the quandary here is that we're getting more contrast with longer exposure times.

With masking, the shadows will build up density first, and this corresponding density will reduce the rate at which more density is formed there. The upper mid-tones will print behind the shadows with regards to time, and the masking effect less extreme here (that is, exposure will be a truer indicator of density, since less masking takes place). The result: darks are held back, upper tones print darker faster = less contrast.

With reciprocity failure, the shadows build density in a relationship that's nearest to Light X Time = Exposure. Under the stronger densities (mid-tones, high-lights) in the negative, we get further & further from this reciprocal relationship because our light value is effectively diminishing. The result is a breakdown of this expression and more light (or time) is required to get the same exposure. The result with regards to contrast is that darks print "normal", and high-lights print proportionally slower = more contrast.