Hi Bryan,
I have the exact same scope and other gear that you are talking about (except the focal reducer and that is on my shopping list in the near future). The 400D has a bulb setting which means you can have the shutter open for as long as you like, the longest default setting is 30 seconds from memory. The 400D also has mirror lock which allows you to open the mirror and avoid mirror shake when taking your photos. You will need to consider a remote switch for this so you do not physically touch the camera when taking the photo (otherwise you will get camera shake).
You will also need to consider an equatorial wedge (also on my shopping list) to allow you to polar align the LX90. This will allow you take exposures longer than about 15 to 20 seconds. I took some photos last weekend and star trailling is noticeable at 20 seconds but minimal at 15 seconds (I use the standard tripod at the moment).
I also notice in your initial post you ask about piggy backing the 400D. You can get a piggy back bracket that is designed for the LX90 so there is no holes to be drilled. You just remove the 3 allen key bolts from the back end of the scope (nothing falls out from inside
), fit the bracket and screw in the longer bolts that come with the bracket. The camera screws onto the bracket like it does a tripod. You will then need to think about lenses but that is a whole new post completely, I use the 60mm macro lens at present it seems to do a reasonable job.
Prime focus is also very achievable. I capture around 1/3 (give or take) of the moon doing this, to give you an idea of the magnification you achieve.
You will need an adaptor which screws onto the camera where the lens would normally go and then another bit screws in there that has an eye piece size fitting that then connects the camera to the scope.
All of the telescope and camera to telescope bits and pieces are all readily available from Bintel in Glebe (I'll give them a plug as they have helped me out so much in my short time star gazing).
Sorry if I have rambled on a bit but there is so much to learn and it is a darn big hill to climb. I hope I have given you some further insight.
Clear skies.