Sunset Shimmer's magical journal is not themost convenient method of communication, at least by modern standards.
It is bulky, it does not have a "mute" setting, it has no spellcheck and, quite importantly, it is finite.
But where magic fails, technology comes to the rescue.
So, new and improved journal:
Consider a robot with two colored pens, that can put a dot in a given point of a journal. Not hard to build with your basic arduino kit.
Connect that to a computer, put the computer online and set up a basic web service, e.g. through the skype API.
Add a cheap webcam and some basic python script with image recognition, APId to the same Skype.
Set up your journal on a table, open on a blank piece of paper, under the web cam and the robots.
Agree with your recipient on a code - a simple one.
Black dot is zero, red dot is a one. You can even have a blank space to be the zero, but thats less robust.
26 letters, a dot, quotation mark, dash, question mark, space and ampersand make 32 symbols, easily encoded in 5 bits. (You can increase the text density further by either agreeing on some form of shorthand for common words or letter combinations, or by using more than two colors, but that is more technically complex. Four-colored pen would allow 64 symbols in 4 bits, and that would cover 32 signs about 15-20% of text worth of most common words and bi- and tri-grams making your text about one third less voluminous, and with a bit of imagination you can probably cut it in half without introducing much ambiguity).
Now with a bit of code and a bunch of publicly available libraries for image recognition, you can send and receive interdimensional messages on your phone (until the page runs out at least), while your journal stays in the security of your flat/room/safe. Moreover, since the dot density is only limited by the resolution of camera and pen bots (and your shorthand code), you can probably fit in much more information than you would by writing normally.
You also get the benefit of spell-check for your messages before you send them, a chance to mute your phone and read the incomming messages later - or using the text-to-speech and speech-to-text pricessing to chat with your favorite Princess while you drive - or run away from the bad guys. You can also send texts to multiple recepients simultaneously (or only to some if you agree how tospecify the addressed's name) or receive it from multiple souces and queue and attribute them so that its readable.
It is also much easier to send copied bits of text if you want to send over, say, texbooks or manuals, without the tedium of rewriting things by hand, or, with a bit of additional not-too-much cleverness also scan and send over simple pictures, if you can achieve sufficient resolution.
If you keep history in the text file you can also search by context, create hyperlinks within text, maintain a table of contents and do a whole load of other useful things with it.
There are downsides to it too. It's not trivial to make a page-turning robot, I think, and if a pen runs out or something breaks, you're out of luck until you go and fix it. It is also not case-sensitive and has no commas or paragrph breaks, unless you're willing to expand your code to allow for it, or have a few clever bits in your program to substitute them back in.
Its alsomuch less convenient for the recipient, since, lacking technology, the Equestrian user would have to use pen, paper and magnifying glass to decode the messages.
Still, the point is, coding is awesome, robots are awesome, and while Friendship is Magic, IT is the next best thing.
PS. I was assuming that the journal transfers colored ink faithfully. If it does not, lines of various angle can serve the same purpose.
Also, the idea is purposefuly held to something that can be done on a budget of fifty bucks and in a few weekends.