First of all I assume that you are not asking if you could run it as a service (then there would be no need to log in) because you say it is an application.
You might be able to run a modified (a custom) init or getty on your system to get your work done. This is just a suggestion. I haven't yet tried something like that. This requires careful implementation since this can affect your whole system very easily.
How you does the it may differ depending on the init system you use. May be you are using SysV init. May be you are using BusyBox
init ot Minit
or something else.
I'd also suggest you to read Karim Yaghmour's Building Embedded Linux Systems. This is an excellent book. No doubt about it. You may find it a lot useful, especially the init sections.