If all the cards are right...
What you are asking about is an adventure. It can go many different ways if you are not prepared with a solid team, understanding for your limitations, and timing. First of all; what craft are you planning on using and what trip length are you looking to do? The Middle/Main combo can be run using different mindsets and game plans.
Kayakers are known to watch for the spring spike and do the middle in 2 days and main in 1 or 2 more. This means, sun up to down paddling of small self contained groups that are hitting specific camps which have shelter or hot springs. Usually eating goo and butter rolled in brown sugar or stopping at the Flying B for breakfast. Better have someone who knows the lines and not plan on scouting anything. No need for a drive to Boundary Creek, the addition on Marsh Creek will be done before brunch unless you screw up.
Cat boaters usually run in groups together early season down marsh and are lightly loaded and willing to portage any trees that may be down from winter. Cat groups are usually formed up of efficient boaters that are lightweight mindset and have their boats rigged and pushing off before I have my coffee in hand. You could assume they resupply at the turn and are in Riggins in 7-10 days. Most groups are eagerly watching the buzz for anyone flying in or the first group's report making the trip second to third week in May.
Rafting is where you need to really question yourself. Seriously stop and think about it. Are you aware enough of what you are about to get into that you can spot the problems ahead of time? Cause if you are recognizing them as they appear, it is too late.
There once was a guy that had a truck with the early version of that tire swap/track system but I haven't seen him since '05 and have no contact info. Boundary summit will open eventually but it is third to forth week in May (except for low snow years which this is not) and you will be looking at paying to play to fly into Indian Creek or rolling the dice and run Marsh. Don't look at the last option lightly. Consider the elevation that you are starting at, that if it is snowing you will have no water and if it is sunny the snow is adding water to a massive watershed and you are about to see the river rise.
Rafters better have a group of kayakers ahead as probes with radios so that if/when they run into wood they can stop the whole group at the somewhat non existing eddies above. Be ready for the rocks to take a toll on your floors, be ready to portage wood, be ready to run Dagger Falls or give up half a day walking gear around, be ready to be cold, be ready for flips and long swims, be ready to sit on the side of the river when it spikes and watch 40' trees float by, and make sure you have extra food.
I am not trying to be MR doom and gloom, and personally love preseason Middle/Main trips, but early on in my rafting I followed someone who said he knew what he was doing. We got snowed on at the put in, had too much gear not enough boats and Marsh destroyed us, we got to Dagger Falls and luckily got out. We waited for the road to open and drove out of boundary which was a saving grace. Looking though the food supply, it was not going to get the group we had to the main. The TL over stated his experience, under prepared, and added many people he didn't know. Since then I have had 3 more marsh experiences and make it a priority to be on the Middle in May. I have found a solid group of boaters that I trust and we keep an open conversation going about safety and when the element are just saying no.
Research other discussions about boundary summit road opening and pay attention to the knowledge and snow depth sites that Carvedog recommends. Highly consider flying into Indian Creek when the air strip opens. Give yourself 14 days to make it to Riggins. The Main is more forgiving but be ready to huge the bank and campsites to be small. Or better yet, ask a merciful permit holder to take you regular season to better understand the elements involved. Talk to Laura H here on the buzz if you have cat boating interest. I could help you with making Idaho kayaking contacts and am more than willing to talk shop.
The reason that permits for the first week of May are still available is that it is extremely difficult to put a situation together where it works.
Early season can be scary high water. I have been on at 7.5 and would personally not feel comfortable above that. Epics happen and people to not return from or to the river.
You seem to be looking for an adventure, try not to make it an epic.