> If someone coukld give me a recipe for assembling and
> linking in a "locally built" startup ...


I'm afraid this is in the manual. Section:

3.12.1 MCS51/DS390 Startup Code

"... . The recommended way would be to copy crt... into the source
directory, adapt it there, then assemble it with asx8051 ... and
when linking your project explicitly specify ... . As a bonus a ..."

An up to date pdf version of the manual is here:
http://sdcc.sf.net/doc/sdccman.pdf
(this might be newer than SDCC version you are using.
The matching version either is on your Windows PC or
should be a few clicks away from http://sdcc.sf.net)





Apart from that I see a few (>=6) other options as well:

b) if your emulator does not allow setting a breakpoint
at locations where no code is available for (strange)
then to save time on single stepping remove global
variables which are zero'ed or initialized.
Then after reset of the emulator single stepping
should get you after a few dozen steps to the point
where main is called.

c) use a simulator instead of your emulator.
Little hardware specific stuff probably is
before main (maybe watchdog, using a paging
register different from P2 on deviates,
enabling/mapping XDATA mem). So a plain
8051 simulator should take you after a few
dozen steps to the point where you think
the problem is.
(let the simulator use the intel hex file
if you really suspect .omf related problems)

d) use one of the cheapo (f.e. Silabs)
in circuit debuggers and do it there.

e) use a disassembler and compare its output
to section 3.12.1. With special care for the address
used here:
lcall _main
(for non commercial readers of this posting an
Open Source disassembler is here:
http://www.8052.com/users/disasm/ )

f) use an on target monitor like paulmon2, cmon51
(google or look into section 6.5 "Related open source tools")
or the Keil monitor to start the SDCC generated
code (compile your source with appropriate --data-loc
and --code-loc).

g) switch on tracing on your emulator (if it allows)
and have a glance over the trace file.

Greetings,
Frieder