I 've got the welder, but I don't know a thing about trikes.   I think some one with a good cad program or a good drawing board and  
and drawing tools can do this.
Tom Nuchols

After looking at those two trike designs - I think I figured out the answer.  No one makes them because they are ugly.  Anyone good at fiberglass?  Cover up those trikes with a nice fiberglass shell and now you are talking!!

From the standpoint of a tadpole....
Larger diameter front wheels reduce the turn-in clearance to the rider; going from 20" to 26" takes about 1/3 more travel through the turning arc.  The track width could be widened to accomodate that with the penalty of higher weight.
 
There would be an increase in rotary inertia.  Not a significant amount but I suspect its enough that it could be felt.
 
Tadpole design adds a bit of complexity to geometry....still not as complex as proper 4 wheel geometry.
 
Scrub radius for example.  To minimize brake steer requires a zero scrub radius.  Imagine a line drawn through the center of the kingpin axis to the ground, at the ground mark that point "A".  Where the tire contacts the ground is point "B".  Zero scrub radius is when these two points are the same.  Taller or shorter tires will move point "A".
 
Granted the kingpin axis can be matched to the tires (and is on Catrikes) but thats only one of the geometry parameters for an example.
 
Suspension systems add a whole new dimension of wizardry.  In selecting a trike RBM had a great variety including the Scorpion FX.  Really cool unit with McPherson struts on the front.  It took about 5 minutes to realize front suspension simply nullifies response along with the bumps.  I prefered the bumps.  
 
Also tested the Catrike Speed, 700, and Expedition (bought the Expedition).   The Speed with 16" fronts felt the most nimble and responsive.  The 700 was a close second although the 700 wheel in the rear seemed to have more flex on turn-in with a slight rebound lateral on turn exit.  The Expedition with 20" fronts and 26" rear also has a higher center of gravity; that gave it a more crisp turn in at the expense of less reserve at the limit.  With both the Speed and 700 as the front grip limits were approaching there was an obvious notification to the rider....and that was the first time I've ever ridden a trike.....The Expedtion also gave notice but at a much lower margin of reserve.
 
At this point I can't se any advantages to increasing front rim size beyond 20", but I could be wrong.

Quote from johnnybent on Nov 20^th, 2011, 5:23pm:

 
I think you should go for it. I'll lend you my welder and metal-working tools. Bring it out to WRL when you finish it. We need a good laugh.