I am seeing more companies using ultra books to their employees.
So those business notebooks cannot be repaired by their corporate helpdesk or IT support department?
They send back to manufacturers to fix if needed?
Usually IT purchases are either through outsource vendor
or direct.
Internal IT helpdesk only escalate the service request to the vendor
if there are any IT assets need to be service.
However this only applies to assets under warranty or under leasing and etc. Yes there are organization which does leasing.
Back to your question.
Is this for your work or home?
If for work, usually any changes or request to IT assets
I highly doubt you can request for specific specs.
Since most buy on bulk purchases which are much cheaper
than a few units.
However depending on organization and hierarchy level, he/she
may request for a specific type of machine.
Home use wise, my advice to you like most have mentioned.
Try to research about the Ultrabook you want.
If the specs you don't like, usually call vendors like Dell
ask whether can order a specific order or not.
Retail shops and etc. they sells specific SKU thus you cannot
configure them.
Most does allow customization expect Asus.
DIY for ultrabook really depends on model, most are not
easy to take apart as it can be dangerous as well as
will void the warranty.
Or if you want to upgrade certain parts like SSD/RAM,
can just bring the part to the service center and ask them
to do it for you.
If you really want something customization than consider a regular notebook. Weight or customization that is up to you to decide.
Hope this helps.