I believe that some of what you propose will accomplish good pest management, and one of these is the dusting of interior wall voids. Perhaps the best dusts to use would be inorganic active ingredients such as diatomaceous earth and silica gel, such as MotherEarth Dust and Drione or Tri-Die. These dusts do kill insects that contact them, they pose extremely low to no hazard to occupants of the homes, and they last essentially forever. Dusting them into wall voids where insects will later wander or nest is an excellent way to kill those insects before they become a bigger problem in the living areas of the homes. Since interior wall voids are probably not going to be filled with insulation you can expect to get that dust throughout the void if you use a power duster that creates air turbulence within the wall during the application. Exterior walls should be insulated, and this poses a bigger problem with injecting dusts and expecting them to go very far, and other options may be better. 

There are some very convenient hand operated power dusters that use a rechargeable battery, and look very much like a cordless drill. These may be good tools for this application. What your customer can expect is to have the wall voids, potentially major highways for many kinds of insects, treated to kill those insects shortly after they enter the voids, and for this to continue to work for many years in the future. Your suggestion that you do a "good inside spray" may not necessarily accomplish anything if there are no pests present, and you might rethink this part. Since the active ingredients that we mix with water to apply will generally last no more than a few weeks, if no bugs are there to kill then you may be doing little more than wasting the insecticide and exposing people to it unnecessarily. 

However, an inspection of the interior will be important in determining whether or not "no" pests are present. During even winter months there are plenty of bugs alive inside. Some of these may be over-wintering house guests, such as cluster flies, ladybugs, ground beetles, stink bugs, and a host of other kinds that find our structures quite suitable as their cozy winter quarters. Locating just where they are hiding and treating that place directly would be very helpful. There also may be carpet beetles infesting food or clothing, stored food pests, silverfish, etc. that remain active inside in the winter, and for some of these you again would need to determine the exact source, deal with the infested material, and perhaps treat locally to pick up stragglers. 

Your final suggestion of doing exclusion work is fabulous, and should be an easy sell to the homeowner. Explaining to them the benefits of keeping pests outside rather than dealing with them once they make it inside should definitely get them on your side and help their decision to pay you to do that exclusion work. Properly done the exclusion materials should be there forever to keep out bats, rats and mice, birds, snakes, and all manner of crawling bugs. This kind of work can be tedious and time consuming, given how porous our homes can be, with plenty of openings around doors and windows, plumbing and electrical access points, around vents and under the eaves, etc. This is an excellent add-on service that will prevent a lot of insecticide spraying in the future. 

There are other great and necessary winter pest control procedures that involve ornamental plants, and if you are set up for power spraying you could consider doing dormant spray applications to many kinds of trees and shrubs. These oils and fungicides kill over-wintering insect eggs, scales, and other stages of the bugs. They help kill fungus spores that cause springtime diseases such as anthracnose, leaf curl, fireblight. If they have dormant fruit trees these are needed applications. You can consider applying systemic insecticides to the soil around trees where aphids and beetles and scales, whiteflies, or other sucking insects are likely to infest those trees in the spring. The products like Merit can be applied in the winter to allow the active ingredient to move up through the tree to be ready to move into the leaves the moment they develop, killing those early insects that attack the first foliage. 

Weed control with pre-emergents may be another add-on service that provides the customer with a better looking lawn or landscape, or prevents the weeds on driveways and other bare-ground areas. These are just some of the many ways to make money and provide the customer with a legitimate service during the traditionally slow months. Thanks for the question.