I’m in a comparable situation to you, working on my PhD in the subject of philosophy (hi, godzillatemple!) besides a full-time job. It’s hard working for a while, but on the other hand a job on the side offers experiences that research doesn’t give. Furthermore it helps you do something useful while inspiration is lacking.
Apos: it is hard for a while to switch back to academia, since you need a different kind of thinking for philosophy. I strongly suggest that you do not completely lose touch during your time in a normal job. I always continued to read philosophy, work on articles (most of which I didn’t finish, but it did keep me involved), give presentations at symposia, discuss with friends. That way, you’ll only have to re-adjust to doing philosophy full-time. Furthermore in that way you may already do some thinking through during your wilderness years.
Back to the original question. A PhD normally only prepares you for the first step in the tenure track, which I assume you are familiar with. This will involve a lot of work (groveling, if you put it negatively) in getting temp jobs teaching and doing research until you have achieved tenure. This preparatory work may at times seem disheartening but from what I understand from friends can be done. If you hate to have to apply constantly for jobs and grants, this is not the road for you. Otherwise it is just part of the job, and most jobs involve some tedious tasks.
Still there will be times when you will not have a grant or position: I’d suggest to try to find a part-time job in your current field. I hope there do exist some functions, maybe not in your current position but possibly as a part-time (or freelance) researcher? It helps to pay the bills and keep you occupied.
After the PhD (and even while working on it) you should work hard at exposure: write articles, go to symposia and conventions, so people know who you are and what you can do. This may help in getting tips about positions, and give you a head start in application procedures.
Another suggestion: you might consider taking a secondary speciailization in business ethics. Since you are familiar in finance, you would have a head start in this area. This could help you get a position teaching or researching this field, and once you have a position somewhere it is easier to branch out to your real interests. Furthermore, you could try to set up a kind of business ethics consultancy (there are others who do this). You might have contacts in the field of finance that could help you here. It would help if you could at least be affiliated to a university (even if without pay).
In the end, tenure is almost a necessity. Writing books may work for some, but most need a basis to start from. Giving lectures and the like doesn’t get you much income. Only if you are really famous you can tour the world, visiting universities on invitation for special chairs.
There are a few people who have gone a different route, turning non-academic publicist, making television and radio programs and writing for the non-professional media. But it is hard to get into that as well.
Good luck!