The Bulbapedia article on the Gracidea states only that you need a fateful encounter Shaymin in your party to get the Gracidea. The Shaymin does not need to be from the Toys 'R Us event (however, that particular Shaymin does have the 'fateful encounter' flag). This is supported by a comment from ChickasaurusGL, who is a well-known researcher.
I found conflicting information on whether a Shaymin that does not have a 'fateful encounter' flag is able to transform to Sky Forme, but it is quite clear that you need a fateful encounter Shaymin to get the Gracidea to begin with.
According to the Bulbapedia article on Shaymin, any Shaymin encountered in Flower Paradise in Platinum will have the fateful encounter flag. However, a Shaymin encountered in Flower Paradise in Diamond/Pearl will not have the 'fateful encounter' flag (source, source).
This is where you run into a problem. If you're playing Pokemon Platinum, you cannot reach Shaymin using the void glitch and tweaking. This is because the void glitch was largely patched in Platinum (source). Furthermore, if you were to reach Flower Paradise some other way (e.g. cheating) in Platinum, then Shaymin will not be there unless you have Oak's Letter (source).
This means that you must obtain Oak's Letter in some way (and follow the usual process on Route 224), or happen upon a fateful encounter Shaymin in a different way. The easiest way to get Oak's Letter is using the DNS exploit allowing you to reach a custom server that hosts old Gen 4/5 events. The process is linked here. If the DNS address doesn't work, check the description of this video for a (hopefully) up to date address. Note that this will not help you if you are playing on an emulator.
