there is no such feature in SAP, is it to much work for the planner to just send a screen shot to the buyer?
you could create a report with ME5A listing all PR for a purchasing group which have a status N and were last changed 5 days ago
change date can be taken from dynamic selections. you can save a selection variant and in the attributes you can define that the date is dynamically calculated.
so you can create a selection variant per purchasing group.
Then you schedule a job via SM36 for daily execution and as spool list receiver you enter the email address of the purchasing group.
So the buyers with open PR for more than 5 days gets the list as email. No specific "Hi Mr xyz, no "you forgot to convert attached PRs", but the effect is just the same, he gets an email, either the buyer reads it or ignores it like he ignored your PR for 5 days.
(I don't think he ignored your PRs,he probably made calls to get prices, to find the right vendor,... or was just on vacation)
A meeting to discuss the matter would probably help more than sending emails for items the buyer finds anyway everyday in his workplace in ME57 or document overview of ME21N . Such emails will just add to his pile of work and will give him even less time to really convert your PRs.