I agree with HuskyPup that, if you build the one-pagers, registration info should be kept open and that there should be no appearances of spamming... but I think that the single page approach inherently looks manipulative. Read the Google Webmaster Guidelines about doorway pages.... [google.com
Doorway pages are typically large sets of poor-quality pages where each page is optimized for a specific keyword or phrase. In many cases, doorway pages are written to rank for a particular phrase and then funnel users to a single destination.
Whether deployed across many domains or established within one domain, doorway pages tend to frustrate users, and are in violation of our webmaster guidelines.
Google gives you some wiggle room here in that they say "poor quality", suggesting that good quality might be OK.
In order to rank at all, though, these one-pagers will need to get links from somewhere. Possibly, if they contain some extraordinarily good and unique information, you might be able to attract natural links to them, enough to overcome Google's aversion to the setup.
Otherwise, the usual temptation is either to interlink them, or to link to them from the main site, or to link to them from common sites you control. With any of these arrangements, though, whether you control the linking or not, it does start to look like a closed link network (something else Google feels is manipulative), which is likely to be algorithmically filtered, if not utimately penalized. If the backlinks into these sites are completely independent and have had a chance to develop over time, then you might be OK.
If you really, really knew what you were doing, you might develop independent sites on the best of your domains, good quality sites that are more than one-pagers, and I feel you'd want to keep them separate and to be extremely prudent about how you used them and when.
I'd recommend not
promoting the domains at all, and simply 301 redirecting them to the main site. No actual manipulation and therefore no appearance of manipulation, but also no SEO benefit and no problems with Google. The SEO benefits are questionable anyway. Hope for some type-in traffic from the domains, and be happy your competitors aren't using them.