It is possible that front-ends my not have enough information to do what they want in the various transaction callbacks. I suggest adding a new element to the pmtrans_t structure - a void *context. The alpm_trans_init() functions would of course have a new parameter to pass the data in. This context element could be anything, but would likely be some sort of structure with additional information. In my case, since I'm using an OOP language to wrap around libalpm, I'd pass in an object, allowing me to interact with it in the callbacks.

Since there are three callbacks, it might be better to be able to provide three different contexts. In such a case it might be neater to refactor the callback registration from the alpm_trans_init() function and have alpm_trans_register_event_callback(...), alpm_trans_register_conv_callback(...), alpm_trans_register_progress_callback(...). This would be beneficial even if the contexts are not implemented, as these parameters are optional (NULL can be used) and the interface is very long, even though this would be more verbose.

Maybe the whole transaction overhaul for universal transactions (FS#9694) would be a good opportunity.