I think I almost climaxed. Was my method safe?

You weren't "about to have an O"; you HAD an O. Sounds good!

To those with experience, does it sound like I was about to have an O?

I agree with @sphinx, congratulations on achieving a dry-O, however I would like to CAUTION you about the possibility of doing yourself DAMAGE or INJURY utilizing the method you described.

Is it safe to use the Aneros in this manner?

IMHO, NO, it isn't. A number of years ago when I was first learning about anal play, prior to my knowledge about the Aneros device, I inadvertently injured my prostate employing what can best be described as a vigorous self massage using a G-spot wand massager. I had a very strong orgasm as a result of that session, but I also got BLOOD in my ejaculate. I was sore for more than a week until I healed. I have since learned to be more respectful of my body and its limitations. I would like to spare you from experiencing a similar pain. Be smart, play safe, you don't have to use extraordinary force or effort to experience these prostate based orgasms, you just need to be tuned into your own body's signals/sensations/feelings. This tuning often takes time for men to learn so be patient and gentle with yourself. The Aneros is designed to be manipulated utilizing the body's musculature ONLY! Applying or employing other forces to increase pressure/stimulation is DANGEROUS! Please heed this warning to prevent yourself from injury.

I don't get this kind of pleasure when using the device in any of the normal positions. Lying on my stomach is the only thing that comes remotely close.

Then this is probably a good starting position for you, learning to "Just Let Go !" is the first lesson to learn. Good Vibes to You !

Thanks rumel. I suspected as much.

@EJGJCO Remember the first rule for Aneros newbies? hands off the penis! Well the second rule is: hands off the Aneros! Listen to @rumel He knows what he is talking about!