My attempts to find this recommendation have so far been unsuccessful. Can you provide links to Ilford's and Kodak's recommendations? I understand that EI 200 with normal dev simply moves the density range further up the HD curve. Looking at Ilford's recommendations for developing HP5+ exposed "inadvertently" at EI less than 250, they suggest developing in their Perceptol Stock for less than normal dev time (in other words, "pull" development - overexpose and under-develop). Granted they also admit that negative quality (however that's defined) will suffer a bit but negs will be usable. I think choice of developer will also influence "negative quality." In their link below, there are these recommendations:
http://www.ilfordphoto.com/Webfiles/20106281054152313.pdf
Page 3 - EI 250, develop time = 13 minutes
Page 5 - EI 200, develop time = 11 minutes
Page 5 - EI 100, develop time = 9 minutes
Also this quote:
Development times may need adjusting to
suit individual processing systems and working
practices. If an established system is producing
good results, adjust the recommended
development times until the desired contrast level is
obtained. My emphasis.