Ok.... this can be really tough. I have worked a offices that have tried this, and many of the usual marketing methods just don't seem to work.
At the high end, so much of the business comes from referrals or agents who live in those areas end up getting the listings.
There are a number of ways to get some attention at that level.
You can do mailers to these areas, but you need to be willing to kick in the extra dollars to make them unique. You want to approach these folks as professionals so start off by telling them about listings you have in your office for lower priced homes, phrasing it as a good investment property. Once you get a response on that level, you have a better chance of getting considered to take the listing in the future for that client.
Surprisingly, one of the best sources of my high end listings was charity work. My partner and I pick a different charity each year to donate 10% of our income too. One year it was the red cross and we ended up getting honored at a banquet. We got several high end listings from other contributors off that. Not a planned benefit, but it says something about doing good things in the community... do good things, and good things will come your way.
This one may sound nuts, but I swear it can work. Have your firm buy a house in the upscale neighborhood. Once you have bought it, put your sign in the front yard with a "Sold By" rider. This is likely to get you some calls just from folks in the area. Next, you can either have the head guy for the office move in or you can start renting it as an executive rental. I don't know if you have those in your area or not, in my area, large corporations will rent homes like these for folks visiting the area for anywhere from a month to years. My home is currently rented in a deal like this while I am in the UK. If you don't want to keep the house, turn around and put it on the market again for enough over what you paid to cover sales costs. This is your chance to show the locals how you sell a home. Put up the sign, but then do an open house that really dazzles the neighbors. Maybe get it catered and have it be by invite. Send invites to all the local homes. You can even have folks checking for invites at the door. Have your agents placed through the home to answer all questions. Your basically buying a home where you want to get listings so that you can dazzle the neighborhood. Get comfortable with not making money, and even losing some in the sale. While it may end up costing you a big pile of dough, it can be the single most effective way of breaking into the upscale market your after. A firm I worked for did this, but would buy a home that could be improved in some way and then would make renovations. They still often lost money on these deals, but they considered it a cost of advertising and ended up getting lots of high end deals out of it.
You might also want to create a sub-brand within your company that advertises itself as servicing higher end home needs.
Take a full color page in the local homes magazine that these more expensive homes tend to dominate.
You can come up with more, but you get the idea.
Some may be surprised by this, but I would not suggest door knocking in these particular neighborhoods. I would suggest delivering materials to their door tho. If these are the types of homes that gates at the entrance, there are usually receptacles for such things on the gate.
If there are clubs, gyms or golf courses associated with communities that have the homes your interested in, get involved there. If they have events, sponsor them.
This can all be a very expensive campaign. But considering the prices of these homes, the cost is not out of line with what your trying to achieve.
R