Ok, here's my reasons (rightly or wrongly) for wanting to do it this way at this stage of my Basecamp learning:
It took me less than a minute to get a route in Google which does exactly what I want for my up coming trip - i.e. avoids motorways and tolls. It also has a good mix of twisty roads going through interesting places.
It took me less than two minutes to transfer that route to Basecamp and convert it. So not really that complicated or time consuming, but of course makes me wholly reliant on the routing god of google!
Using Basecamp the the problem I have right now is that the avoidances are set up for the US style of roads (e.g. State, interstate etc.) and I'm finding it difficult to get Basecamp to suggest a route as good as the google one.
So not a major issue as I can do exactly what you have suggested (and I have done this for routes in the UK) and simply sit there and drag the route about a bit and hope I don't do fall foul to putting the way point on the wrong side of the road and get sent up some side road to do a u-turn!
However, this is my first trip abroad on a motorbike for nearly 20 years (yes sad I know) and I don't really have much experience of what the different categories of road will be like to ride on. I also have 14 routes to plan in a short period of time and I'm not sure my skills are quite enough to sit there and meticulously plan them all to the detail required.
So given how quickly I can get a suggested route on google which seems to be a good balance of mileage avoiding tolls and motorways and throws in good twisty roads, this just seems to be the best interim solution until I can get more hours on Basecamp and fully understand how to use it to do exactly what I want.