Save any RAW or bitmap format as a DNG file with a lossy option.
Edit: we don't support this from the "save as" menu - see below.
Sorry, I was slightly confused about what you were trying to do. We can use Adobe to convert them. Install Adobe DNG Converter and make sure you use the "Pick DNG Converter..." if you don't already have it set up in preferences. It's not part of the Save As... menu, but under Tools>Convert RAW to DNG. By default it's set as lossy (compressed). You may want to back up your original RAW files first.
I don't think Adobe DNC Converter supports converting TIFF files yet, which means we can't easily add that support either. Let me know if I'm wrong about that. Their documentation suggests it only converts RAW files to DNG. It's possible that there may be some other third party desktop app that supports it, in which case it might be possible to use it in place of Adobe DNG, but so far I'm only seeing cloud-based apps.
What i had hoped was that the DNG SDK would provide the info you'd need for this to be done. I can't see any software that can do what I'm asking aside from ACR and LR. But there's not much enthusiasm for lossy DNG from what I've read. So I had assumed this was the reason nobody else bothered to implement the option for it in their DNG output. From what I can work out if you want to send someone either a raw file or a very high quality image with a low file size from one place in the world to another (a core function of PM), or if like me you've got your original raw and master tiff files being backed up on site but you need an archival file that is much smaller for off site backup via internet transfers.
There used to be an app called toDNG for iOS that converted tiffs and other bitmap files to linear DNGs, so clearly it's possible. Maybe referring to the SDK would be useful. I'm not a programmer so I can't help.
I'm sure it's theoretically possible, but at the moment we're using DNG Converter for the conversions, so doing it in another way would involve more programming effort. It may be something we look at in the future, but it's not in our immediate plans.