Here are some notes concerning netbsd/amiga shell and Xamiga environment:
(sorry for those who already read that .. ive posted it some time ago)

Should be interesting for you : 

For a good start let me say sth. about display modes with shell
environment (set in kernel)... (Im not sure if Xdaniver
relies in some way on these but it seems it does) 

AFAIK there is ntsc and pal modes support in kernel and that 
display automatically recognizes only these both.
If u select <= 400 height NTSC is default otherwise ( e.g. 512 )
PAL.   

$ iteconfig -h 512   (to turn into pal)

Moreover u can select -A switch in loadbsd command line which is 
considered to replace PAL and NTSC with DblPAL and DblNTSC respectively..

Dunno if there is other display modes support for ECS/AGA chips (/dev/grf0
 -->  /dev/ttye0)

It seems that Xdaniver relies on Vsync and Hsync for these modes set in
kernel. (At least for me: when i changed to 31.5 kHz (before was < 30) in
kernel and switched to pal got stable display ... 
before it was flickering and jerking) -> this is due to my VGA monitor.

However u can change all parameters of selected mode (at least with DblPAL 
and DblNTSC on AGA machine) including vertical and horizontal frequencies
(e.g. for decent VGA monitors which cant handle standard kernel Dblpal and
Dblntsc modes)

In order to do this u need to recompile the kernel (tough) or just
use binpatch for existing kernel image under AmigaOS. (easy :-)

> binpatch -s string=value kernel_image

where string is : _AGA_vtotal
	          _AGA_htotal 
                  _AGA_vsstrt
		  _AGA_vsstop
	          _AGA_vbstop
		  _AGA_vbstrt
		  ...

 or e.g.  _AGA_hcenter
These are for DblNTSC mode . Add "p" as a prefix for DblPal mode:
	      like:  _pAGA_vtotal 
		     _pAGA_htotal  etc.

U can use any text editor for getting names of these strings:
just load kernel and search for _AGA* and _pAGA 

U can get values e.g from Moned util for amigados (probly on aminet)
Sometimes it takes time to adjust for the best results.
(Hopefully i was able to get 720 x 540 Dblpal on my decent vga
monitor)
As refers to Xdaniver i must say that before making any changes (listed
above) i wasnt able to run Xdaniver in DblPal mode.

Now any of

$ X -w 720 -h 540 
works perfectly and ive got a stable display in DBLPal mode. 


Switching below 400 in height will get u DblNTSC without any flickering.

Greetings
		gelo.