Depends on the relay race. Yes, for most legs with a good change over and rolling start, you need to add about 0.7 to get an equivalent open time for 400. Coe's fastest split was 45.5, worth around 46.0 - 46.2. But in the match v USA at Crystal Palace in June 81, he ran a leg in 45.6, where he took the baton 2m up, stumbled, practically hobbled through the start/finish line and by this time was about 5m down on the US athlete. He then receives no drafting, runs slightly wide to get past in the home straight, before easing down 20m from the finish. This is on YouTube, and clearly represents the equivalent of a sub 46.0 open. He also had a FAT of 11.01 in a spring meet for 100m on cinders. And a pb of 21.6 for 200m, which is the last in a set of 6 200m sessions with short recoveries. Yes, this is hand timed, but represents sub 22 ability for 200m. I can't check at moment, but i seem to recall he went through 300 in 33. During his 46.8 open.
Overall, there is enough evidence there to convince me that he could have run a sub 11 for an open 100 from blocks in 81. And that was the year he broke the mile WR and won the British 3000m title indoors.
As for Rudisha, he is clearly capable of 45 flat for 400, although his change of pace is not as good as Coe's, and as he is so tall, I don't think he'd be much faster, if at all, than Coe over 100m from blocks.
400, Yes, co I certainly don't think Rudisha could run 10.5 even with some practise using blocks. I'd put him and Coe around 10.8 for 100m.