I appreciate how you are trying to find a solution which is cross-platform: Windows, Mac and Linux. But you're stuck because you don't have a decent ext4 utility for Windows and Mac. Fair enough. Here's a possible trick to get around that.
Oracle VirtualBox IS cross-platform. Why not make a VirtualBox Virtual Machine with some sort of Linux Distro as the guest OS (perhaps some really minimal, small one). Your Picard utility would be the only thing that it performs, when it's booted (no time wasted with creation of users, installing software, etc. in that VM's OS). The ext4-related utilities Picard needs are easily available in the Linux Guest OS. Hopefully, that VM can take over control of the SD Card to the extent necessary for Picard to "do its magic".
This way, you're only developing your utility for one platform (the VM's guest OS), and not three (Windows, Mac, and Linux). Then you post the Picard VM for download.
Sure, people would have to do a few extra steps to download and install VirtualBox, but it's dead easy for them, and would make your life alot easier, since you wouldn't be stuck in your current "dead end". In fact the Raspberry Pi-realted Video tutorials for installing Virtualbox already exists.
This idea is simply just using VirtualBox for yet one more purpose: Picard running inside a Linux Guest VM.