Asking for PhD admission in a different faculty than potential supervisor
There is no general answer. You should check the university's regulations. Additionally, you should talk with your potential supervisor about the issue. Even if it's not allowed, he might know a way to circumvent it.

Moreover, I think that the title itself is secondary. The most important part is what you did during your PhD. (To illustrate the point: after that PhD-project, nobody will hire you to design a car engine just because you have a PhD in mechanical engineering.)

If employers have a job opening, they are looking for somebody with the skills necessary for the job. Typical job offers usually include the phrase "desired qualifications: a PhD in computer science or something equivalent." This translates to "we think that somebody with the necessary skills will have a degree in computer science but if you possess these skills and a different PhD, we still want you to apply."

The place to present your software development skills is your cover letter and your CV/resume once you apply for positions. Make there very clear why you are the perfect fit for the position. For instance, in the CV, you should list the software you developed and the languages you programmed in (with number of years you are using them); and in the cover letter, you explain how these things qualify you for the position.