There was cyanotype + salt print process I experimented with a few years ago with some success.
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I separated the original negative into two; one for the salt print and another for the cyanotype then proceeded as follows:
1. Expose, process, and dry the salt print as usual.
2. Sensitize the print with cyanotype solution. Note: This will bleach out the silver image due to the ferricyanide.
3. Expose and process the cyanotype as usual.
4. Brush a dilute developer solution (I used HC-110) onto the bleached silver areas to redevelop the silver.
You can see an area in the lower right hand corner where I failed to redevelop a portion of the silver.
The alkaline nature of the developer causes bleaching of the cyanotype so its a bit of a balancing act. A slightly acidic amidol developer might work for tray processing but the paper would probably stain.
Coincidentally, last couple of days I abadnoned my regular cyanotype work - no doubt inspired by the original premise of this tread, and jumped back into something I had started long time ago but didn't follow up as usual - namely, blue toning of POP and salt prints. I was going to start a separate thread, but may be I can describe the basic concepts here.
You don't have to do the actual cyanotype over the salt or VDB - you can get chemically identical results but with much better control and much ease (tray processing) if you follow the path of classic blue toning process and then revive the silver. First part is relatively easy, the last part as you are reporting is tricky.
Here is how I am approaching it:
Step A: Bleach + blue-tone salt print in K Ferri + FAC + KBr mixture.
Si converts to Si ferrOcyanide releasing K FerrOcyanide
K FerrOcyanide combines with KFerri to form Prussian blue
Si ferrOcyanide converts to silver bromide releasing one more K FerrOcyanide which further makes Prussian blue
Print is now all blue, having Silver been converted to pale yellow bromide
Step B: Convert silver bromide to image-forming silver.
B1) Use indirect toning - take your pick. I am looking at thiourea carbonate as well as home concoction of Black salt I have used in the past to tone salt prints.
B2) Develop - Bring back silver metal by using a reducing developer such as the regular paper developer as you are using. I don't have the traditional darkroom chemicals. I am wondering if a simple (ascorbic acid + base) developer will suffice, which I have plenty of. [I am also wondering if an alkali can be simply taken out of a developer as its function (as I understand) is to accelerate and increase contrast - latter is important if you are developing a print from a latent image. What we are doing here is simply flash expose and develop to the same degree all over. Perhaps the next step can be eliminated, if that is the case.]
If an alkaline developer is used, the blue will be bleached by the basic nature of both the developer (may be amidol is an exception) and toner, essentially forming yellow ferric hydroxide.
Step C: Regeneration of Prussian blue.
Treat the paper with K ferrOcyanide.
Ferric hydroxide reacts with K ferrO to form Prussian blue.
Now, in theory, your starting 1 atom of silver results into 1 atom of silver (or silver sulfide or combination) and if the reactions are efficiently performed 1 molecule of ferric ferrocyanide or Prussian blue.
:Niranjan.