guid
doesn’t affect the image URL, it is just used as a unique identifier. It could be random gobbledygook and as long as it is unique it wouldn’t make a difference. It should be safe to use postmeta as long as you aren’t using any plugins that would potentially completely overwrite all postmeta associated with an attachment – no plugins I am familiar with would do so, just mentioning what you would need to check on. Most plugins use update_post_meta
which adds or updates if a particular postmeta key is found, so use a unique meta_key
and you should be safe.